【Docker】Dockerfile 最佳实践-ENV

参考教程：https://docs.docker.com/develop/develop-images/dockerfile_best-practices/

环境

1. virtual box 6.1
2. centos 7.8
3. docker 19.03

ENV

To make new software easier to run, you can use ENV to update the PATH environment variable for the software your container installs. For example, ENV PATH=/usr/local/nginx/bin:$PATH ensures that CMD ["nginx"] just works.

为了使新软件易于运行，您可以使用 ENV 来更新容器安装的软件的 PATH 环境变量。例如，ENV PATH=/usr/local/nginx/bin:$PATH 确保 CMD ["nginx"] 正常工作。

The ENV instruction is also useful for providing required environment variables specific to services you wish to containerize, such as Postgres’s PGDATA.

ENV 指令还可用于提供特定于您希望容器化的服务的必需环境变量，例如 Postgres 的 PGDATA。

Lastly, ENV can also be used to set commonly used version numbers so that version bumps are easier to maintain, as seen in the following example:

最后，ENV 也可以用来设置常用的版本号，以便更容易维护版本，如以下示例所示：

ENV PG_MAJOR=9.3
ENV PG_VERSION=9.3.4
RUN curl -SL https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH=/usr/local/postgres-$PG_MAJOR/bin:$PATH

Similar to having constant variables in a program (as opposed to hard-coding values), this approach lets you change a single ENV instruction to auto-magically bump the version of the software in your container.

类似于在程序中具有变量（与硬编码值相反），这种方法使您可以更改单个 ENV 指令以自动神奇地修改容器中软件的版本。

Each ENV line creates a new intermediate layer, just like RUN commands. This means that even if you unset the environment variable in a future layer, it still persists in this layer and its value can’t be dumped. You can test this by creating a Dockerfile like the following, and then building it.

每条 ENV 行都会创建一个新的中间层，就像 RUN 命令一样。这意味着，即使您在以后的层中取消设置环境变量，它也仍然保留在该层中，并且其值也无法转储。您可以通过创建如下所示的 Dockerfile，然后对其进行构建来进行测试。

FROM alpine
ENV ADMIN_USER="mark"
RUN echo $ADMIN_USER > ./mark
RUN unset ADMIN_USER

$ docker run --rm test sh -c 'echo $ADMIN_USER'

mark

To prevent this, and really unset the environment variable, use a RUN command with shell commands, to set, use, and unset the variable all in a single layer. You can separate your commands with ; or &&. If you use the second method, and one of the commands fails, the docker build also fails. This is usually a good idea. Using as a line continuation character for Linux Dockerfiles improves readability. You could also put all of the commands into a shell script and have the RUN command just run that shell script.

为了避免这种情况，并真正取消设置环境变量，请在外壳程序中使用带有外壳命令的 RUN 命令来设置，使用和取消设置该变量。您可以使用 ; 或 && 分隔命令。如果您使用第二种方法，并且其中一个命令失败，则 docker build 也会失败。这通常是个好主意。将 用作 Linux Dockerfiles 的行继续符可提高可读性。您也可以将所有命令放入一个 shell 脚本中，并让 RUN 命令运行该 shell 脚本。

FROM alpine
RUN export ADMIN_USER="mark" 
    && echo $ADMIN_USER > ./mark 
    && unset ADMIN_USER
CMD sh

$ docker run --rm test sh -c 'echo $ADMIN_USER'

总结

介绍了 Dockerfile 的 ENV 指令的最佳实践。