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

環境

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"]

為了使新軟體易於執行，您可以使用 ENV 來更新容器安裝的軟體的 PATH 環境變數。例如，ENV PATH=/usr/local/nginx/bin:$PATH 確保 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 指令還可用於提供特定於您希望容器化的服務的必需環境變數，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:

最後，ENV 也可以用來設定常用的版本號，以便更容易維護版本，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.

類似於在程式中具有變數（與硬編碼值相反），這種方法使您可以更改單個 ENV 指令以自動神奇地修改容器中軟體的版本。

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.

每條 ENV 行都會建立一個新的中間層，就像 RUN 命令一樣。這意味著，即使您在以後的層中取消設定環境變數，它也仍然保留在該層中，並且其值也無法轉儲。您可以通過建立如下所示的 Dockerfile，然後對其進行構建來進行測試。

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.

為了避免這種情況，並真正取消設定環境變數，請在外殼程式中使用帶有外殼命令的 RUN 命令來設定，使用和取消設定該變數。您可以使用 ; 或 && 分隔命令。如果您使用第二種方法，並且其中一個命令失敗，則 docker build 也會失敗。這通常是個好主意。將 \ 用作 Linux Dockerfiles 的行繼續符可提高可讀性。您也可以將所有命令放入一個 shell 指令碼中，並讓 RUN 命令執行該 shell 指令碼。

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'

總結

介紹了 Dockerfile 的 ENV 指令的最佳實踐。