为什么在FROM中之后在Dockerfile中不能再次使用build arg? [英] Why can't I use the build arg again after FROM in a Dockerfile?

问题描述

我正在使用Docker 18.05.0~ce~3-0~ubuntu，我想将构建参数传递给FROM和Dockerfile中的其他行.您希望以下代码可以正常工作:

I'm using Docker 18.05.0~ce~3-0~ubuntu and I'd like to pass a build argument to the FROM as well as other lines in my Dockerfile. You would expect the below to work:

ARG FROM_IMAGE=ubuntu:bionic
FROM $FROM_IMAGE

COPY sources_list/$FROM_IMAGE /etc/apt/sources.list

它适用于第二行(FROM)，但其行为就像未在COPY行中设置一样:

It works for the second line (FROM), but it behaves like it is unset in the COPY line:

第1/3步:ARG FROM_IMAGE = ubuntu:bionic 第2/3步:从$ FROM_IMAGE开始 ---> 8626492fecd3 [...] 步骤3/3:COPY sources_list/$ {SOURCES_LIST_FILE}/etc/apt/sources.list 复制文件失败:复制目录失败:mkdir /var/lib/docker/overlay2/0536b4e280ddca2fec18db9d79fa625a8be86efdbaaea5b3dbbefcdaaab3f669/merged/etc/apt/sources.list: 不是目录

Step 1/3 : ARG FROM_IMAGE=ubuntu:bionic Step 2/3 : FROM $FROM_IMAGE ---> 8626492fecd3 [...] Step 3/3 : COPY sources_list/${SOURCES_LIST_FILE} /etc/apt/sources.list failed to copy files: failed to copy directory: mkdir /var/lib/docker/overlay2/0536b4e280ddca2fec18db9d79fa625a8be86efdbaaea5b3dbbefcdaaab3f669/merged/etc/apt/sources.list: not a directory

如果添加另一个单独的生成arg，则它适用于同一COPY行:

If add another, separate build arg, it works for the same COPY line:

ARG FROM_IMAGE=ubuntu:bionic
FROM $FROM_IMAGE

ARG SOURCES_LIST_FILE
COPY sources_list/${SOURCES_LIST_FILE} /etc/apt/sources.list

第4/4步:COPY sources_list/$ {SOURCES_LIST_FILE}/etc/apt/sources.list ---> 7f974fffe929

Step 4/4 : COPY sources_list/${SOURCES_LIST_FILE} /etc/apt/sources.list ---> 7f974fffe929

为什么在FROM行上和之后不能两次使用FROM_IMAGE build arg?我找不到任何记录在案的这种限制.

Why can't I use the FROM_IMAGE build arg twice, on and after a FROM line? I fail to find any documented restriction of this sort.

推荐答案

为什么在FROM行之后以及之后，我不能两次使用FROM_IMAGE build arg?

Why can't I use the FROM_IMAGE build arg twice, on and after a FROM line?

取决于将ARG与FROM行相关的放置位置，确实存在实际差异:

There is a real difference depending on where you put ARG related to FROM line:

• 第一个FROM之前的任何ARG都可以在任何FROM行中使用
• 在构建阶段(在FROM之后)内的任何ARG都可以在该构建阶段使用

这与构建阶段的机制有关，可以在此处找到一些实际行为参考: https://github.com/docker/cli/pull/333 ，以及有关为何文档和构建机制在ARG使用上有些混乱的讨论在这里:

This is related to build stages mechanics and some reference of actual behavior can be found here: https://github.com/docker/cli/pull/333, and a discussion on why documentation and build mechanics is a bit confusing on ARG usage is here: https://github.com/moby/moby/issues/34129

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