在为Python项目构建Docker映像时，如何避免重新安装包？

Question

我的Dockerfile有点像

FROM my/base

ADD . /srv
RUN pip install -r requirements.txt
RUN python setup.py install

ENTRYPOINT ["run_server"]

每次我构建一个新映像时，都必须重新安装依赖项，这在我所在的地区可能会非常缓慢

对于已安装的cache软件包，我想到的一种方法是使用以下较新的映像覆盖my/base映像：

docker build -t new_image_1 .
docker tag new_image_1 my/base

因此，下次使用此Dockerfile构建时，my/base已经安装了一些软件包

但这种解决方案有两个问题：

1. 并非总是可以覆盖基础图像
2. 随着新图像的层叠，基础图像越来越大

那么，我可以用什么更好的方法来解决这个问题呢

编辑：

有关我机器上docker的一些信息：

☁  test  docker version
Client version: 1.1.2
Client API version: 1.13
Go version (client): go1.2.1
Git commit (client): d84a070
Server version: 1.1.2
Server API version: 1.13
Go version (server): go1.2.1
Git commit (server): d84a070
☁  test  docker info
Containers: 0
Images: 56
Storage Driver: aufs
 Root Dir: /var/lib/docker/aufs
 Dirs: 56
Execution Driver: native-0.2
Kernel Version: 3.13.0-29-generic
WARNING: No swap limit support

Answer 1

为了尽量减少网络活动，您可以将pip指向主机上的缓存目录

运行docker容器，将主机的pip缓存目录绑定装入容器的pip缓存目录docker run命令应如下所示：

docker run -v $HOME/.cache/pip-docker/:/root/.cache/pip image_1

然后在Dockerfile中，将需求作为ENTRYPOINT语句（或CMD语句）的一部分安装，而不是作为RUN命令安装。这很重要，因为（如注释中所指出的）在映像构建期间（当执行RUN语句时），挂载不可用。Docker文件应如下所示：

FROM my/base

ADD . /srv

ENTRYPOINT ["sh", "-c", "pip install -r requirements.txt && python setup.py install && run_server"]

Answer 2

尝试构建一个Dockerfile，其外观如下所示：

FROM my/base

WORKDIR /srv
ADD ./requirements.txt /srv/requirements.txt
RUN pip install -r requirements.txt
ADD . /srv
RUN python setup.py install

ENTRYPOINT ["run_server"]

Docker将在pip安装期间使用缓存，只要您不对requirements.txt进行任何更改，而不管.处的其他代码文件是否已更改。这里有一个例子

---

下面是一个简单的Hello, World!程序：

$ tree
.
├── Dockerfile
├── requirements.txt
└── run.py   

0 directories, 3 file

# Dockerfile

FROM dockerfile/python
WORKDIR /srv
ADD ./requirements.txt /srv/requirements.txt
RUN pip install -r requirements.txt
ADD . /srv
CMD python /srv/run.py

# requirements.txt
pytest==2.3.4

# run.py
print("Hello, World")

docker build的输出：

Step 1 : WORKDIR /srv
---> Running in 22d725d22e10
---> 55768a00fd94
Removing intermediate container 22d725d22e10
Step 2 : ADD ./requirements.txt /srv/requirements.txt
---> 968a7c3a4483
Removing intermediate container 5f4e01f290fd
Step 3 : RUN pip install -r requirements.txt
---> Running in 08188205e92b
Downloading/unpacking pytest==2.3.4 (from -r requirements.txt (line 1))
  Running setup.py (path:/tmp/pip_build_root/pytest/setup.py) egg_info for package pytest
....
Cleaning up...
---> bf5c154b87c9
Removing intermediate container 08188205e92b
Step 4 : ADD . /srv
---> 3002a3a67e72
Removing intermediate container 83defd1851d0
Step 5 : CMD python /srv/run.py
---> Running in 11e69b887341
---> 5c0e7e3726d6
Removing intermediate container 11e69b887341
Successfully built 5c0e7e3726d6

让我们修改run.py：

# run.py
print("Hello, Python")

再次尝试构建，以下是输出：

Sending build context to Docker daemon  5.12 kB
Sending build context to Docker daemon 
Step 0 : FROM dockerfile/python
---> f86d6993fc7b
Step 1 : WORKDIR /srv
---> Using cache
---> 55768a00fd94
Step 2 : ADD ./requirements.txt /srv/requirements.txt
---> Using cache
---> 968a7c3a4483
Step 3 : RUN pip install -r requirements.txt
---> Using cache
---> bf5c154b87c9
Step 4 : ADD . /srv
---> 9cc7508034d6
Removing intermediate container 0d7cf71eb05e
Step 5 : CMD python /srv/run.py
---> Running in f25c21135010
---> 4ffab7bc66c7
Removing intermediate container f25c21135010
Successfully built 4ffab7bc66c7

如上所示，这次docker在构建期间使用缓存。现在，让我们更新一下requirements.txt：

# requirements.txt

pytest==2.3.4
ipython

以下是docker build的输出：

Sending build context to Docker daemon  5.12 kB
Sending build context to Docker daemon 
Step 0 : FROM dockerfile/python
---> f86d6993fc7b
Step 1 : WORKDIR /srv
---> Using cache
---> 55768a00fd94
Step 2 : ADD ./requirements.txt /srv/requirements.txt
---> b6c19f0643b5
Removing intermediate container a4d9cb37dff0
Step 3 : RUN pip install -r requirements.txt
---> Running in 4b7a85a64c33
Downloading/unpacking pytest==2.3.4 (from -r requirements.txt (line 1))
  Running setup.py (path:/tmp/pip_build_root/pytest/setup.py) egg_info for package pytest

Downloading/unpacking ipython (from -r requirements.txt (line 2))
Downloading/unpacking py>=1.4.12 (from pytest==2.3.4->-r requirements.txt (line 1))
  Running setup.py (path:/tmp/pip_build_root/py/setup.py) egg_info for package py

Installing collected packages: pytest, ipython, py
  Running setup.py install for pytest

Installing py.test script to /usr/local/bin
Installing py.test-2.7 script to /usr/local/bin
  Running setup.py install for py

Successfully installed pytest ipython py
Cleaning up...
---> 23a1af3df8ed
Removing intermediate container 4b7a85a64c33
Step 4 : ADD . /srv
---> d8ae270eca35
Removing intermediate container 7f003ebc3179
Step 5 : CMD python /srv/run.py
---> Running in 510359cf9e12
---> e42fc9121a77
Removing intermediate container 510359cf9e12
Successfully built e42fc9121a77

注意docker在pip安装期间是如何不使用缓存的。如果不起作用，请检查docker版本

Client version: 1.1.2
Client API version: 1.13
Go version (client): go1.2.1
Git commit (client): d84a070
Server version: 1.1.2
Server API version: 1.13
Go version (server): go1.2.1
Git commit (server): d84a070

Answer 3

我知道这个问题已经有了一些流行的答案。但是有一种更新的方法可以为包管理器缓存文件。我认为在将来，当BuildKit变得更加标准时，这可能是一个很好的答案

截至Docker 18.09，对BuildKit有实验性支持。BuildKit在Dockerfile中添加了对一些新功能的支持，包括experimental support for mounting external volumes到RUN步骤中。这允许我们为$HOME/.cache/pip/之类的东西创建缓存

我们将使用以下requirements.txt文件作为示例：

Click==7.0
Django==2.2.3
django-appconf==1.0.3
django-compressor==2.3
django-debug-toolbar==2.0
django-filter==2.2.0
django-reversion==3.0.4
django-rq==2.1.0
pytz==2019.1
rcssmin==1.0.6
redis==3.3.4
rjsmin==1.1.0
rq==1.1.0
six==1.12.0
sqlparse==0.3.0

PythonDockerfile的典型示例可能如下所示：

FROM python:3.7
WORKDIR /usr/src/app
COPY requirements.txt /usr/src/app/
RUN pip install -r requirements.txt
COPY . /usr/src/app

使用DOCKER_BUILDKIT环境变量启用BuildKit后，我们可以在大约65秒内构建未缓存的pip步骤：

$ export DOCKER_BUILDKIT=1
$ docker build -t test .
[+] Building 65.6s (10/10) FINISHED                                                                                                                                             
 => [internal] load .dockerignore                                                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                                                                                       0.0s
 => => transferring dockerfile: 120B                                                                                                                                       0.0s
 => [internal] load metadata for docker.io/library/python:3.7                                                                                                              0.5s
 => CACHED [1/4] FROM docker.io/library/python:3.7@sha256:6eaf19442c358afc24834a6b17a3728a45c129de7703d8583392a138ecbdb092                                                 0.0s
 => [internal] load build context                                                                                                                                          0.6s
 => => transferring context: 899.99kB                                                                                                                                      0.6s
 => CACHED [internal] helper image for file operations                                                                                                                     0.0s
 => [2/4] COPY requirements.txt /usr/src/app/                                                                                                                              0.5s
 => [3/4] RUN pip install -r requirements.txt                                                                                                                             61.3s
 => [4/4] COPY . /usr/src/app                                                                                                                                              1.3s
 => exporting to image                                                                                                                                                     1.2s
 => => exporting layers                                                                                                                                                    1.2s
 => => writing image sha256:d66a2720e81530029bf1c2cb98fb3aee0cffc2f4ea2aa2a0760a30fb718d7f83                                                                               0.0s
 => => naming to docker.io/library/test                                                                                                                                    0.0s

现在，让我们添加实验头并修改RUN步骤以缓存Python包：

# syntax=docker/dockerfile:experimental

FROM python:3.7
WORKDIR /usr/src/app
COPY requirements.txt /usr/src/app/
RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt
COPY . /usr/src/app

现在继续进行另一个构建。这应该需要同样的时间。但这一次，它正在我们新的缓存装载中缓存Python包：

$ docker build -t pythontest .
[+] Building 60.3s (14/14) FINISHED                                                                                                                                             
 => [internal] load build definition from Dockerfile                                                                                                                       0.0s
 => => transferring dockerfile: 120B                                                                                                                                       0.0s
 => [internal] load .dockerignore                                                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                                            0.0s
 => resolve image config for docker.io/docker/dockerfile:experimental                                                                                                      0.5s
 => CACHED docker-image://docker.io/docker/dockerfile:experimental@sha256:9022e911101f01b2854c7a4b2c77f524b998891941da55208e71c0335e6e82c3                                 0.0s
 => [internal] load .dockerignore                                                                                                                                          0.0s
 => [internal] load build definition from Dockerfile                                                                                                                       0.0s
 => => transferring dockerfile: 120B                                                                                                                                       0.0s
 => [internal] load metadata for docker.io/library/python:3.7                                                                                                              0.5s
 => CACHED [1/4] FROM docker.io/library/python:3.7@sha256:6eaf19442c358afc24834a6b17a3728a45c129de7703d8583392a138ecbdb092                                                 0.0s
 => [internal] load build context                                                                                                                                          0.7s
 => => transferring context: 899.99kB                                                                                                                                      0.6s
 => CACHED [internal] helper image for file operations                                                                                                                     0.0s
 => [2/4] COPY requirements.txt /usr/src/app/                                                                                                                              0.6s
 => [3/4] RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt                                                                                  53.3s
 => [4/4] COPY . /usr/src/app                                                                                                                                              2.6s
 => exporting to image                                                                                                                                                     1.2s
 => => exporting layers                                                                                                                                                    1.2s
 => => writing image sha256:0b035548712c1c9e1c80d4a86169c5c1f9e94437e124ea09e90aea82f45c2afc                                                                               0.0s
 => => naming to docker.io/library/test                                                                                                                                    0.0s

大约60秒。类似于我们的第一个构建

对requirements.txt进行一个小更改（例如在两个包之间添加新行），以强制缓存失效并再次运行：

$ docker build -t pythontest .
[+] Building 15.9s (14/14) FINISHED                                                                                                                                             
 => [internal] load build definition from Dockerfile                                                                                                                       0.0s
 => => transferring dockerfile: 120B                                                                                                                                       0.0s
 => [internal] load .dockerignore                                                                                                                                          0.0s
 => => transferring context: 2B                                                                                                                                            0.0s
 => resolve image config for docker.io/docker/dockerfile:experimental                                                                                                      1.1s
 => CACHED docker-image://docker.io/docker/dockerfile:experimental@sha256:9022e911101f01b2854c7a4b2c77f524b998891941da55208e71c0335e6e82c3                                 0.0s
 => [internal] load build definition from Dockerfile                                                                                                                       0.0s
 => => transferring dockerfile: 120B                                                                                                                                       0.0s
 => [internal] load .dockerignore                                                                                                                                          0.0s
 => [internal] load metadata for docker.io/library/python:3.7                                                                                                              0.5s
 => CACHED [1/4] FROM docker.io/library/python:3.7@sha256:6eaf19442c358afc24834a6b17a3728a45c129de7703d8583392a138ecbdb092                                                 0.0s
 => CACHED [internal] helper image for file operations                                                                                                                     0.0s
 => [internal] load build context                                                                                                                                          0.7s
 => => transferring context: 899.99kB                                                                                                                                      0.7s
 => [2/4] COPY requirements.txt /usr/src/app/                                                                                                                              0.6s
 => [3/4] RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt                                                                                   8.8s
 => [4/4] COPY . /usr/src/app                                                                                                                                              2.1s
 => exporting to image                                                                                                                                                     1.1s
 => => exporting layers                                                                                                                                                    1.1s
 => => writing image sha256:fc84cd45482a70e8de48bfd6489e5421532c2dd02aaa3e1e49a290a3dfb9df7c                                                                               0.0s
 => => naming to docker.io/library/test                                                                                                                                    0.0s

只有16秒

我们得到了这种加速，因为我们不再下载所有的Python包。它们由包管理器缓存（本例中为pip），并存储在缓存卷装载中。卷装载提供给运行步骤，以便pip可以重用我们已经下载的包这发生在任何Docker层缓存之外

在更大的{}上，收益应该更好

注:

• 这是一种实验性的Dockerfile语法，应该这样对待。目前，您可能不希望在生产中使用此功能进行构建
• BuildKit工具在Docker Compose或其他直接使用Docker API的工具下不起作用。从1.25.0开始，Docker Compose中就支持此功能。见How do you enable BuildKit with docker-compose?
• 目前，托管缓存没有任何直接接口。当您执行docker system prune -a时，它将被清除

希望这些特性将成为Docker for building的一部分，BuildKit将成为默认设置。如果发生这种情况，我将尝试更新此答案