将 ARG 放在 Dockerfile 顶部会阻止层重用吗? [英] Does putting ARG at top of Dockerfile prevent layer re-use?
问题描述
如果在 Dockerfile 顶部声明的 ARG 发生更改，但其值仅用于 Dockerfile 末尾附近的 RUN 命令，Docker 是从头开始重建整个映像还是能够重新使用相关 RUN 命令之前的中间图像?
If an ARG that is declared at the top of a Dockerfile gets changed, but its value is only used for a RUN command near the end of the Dockerfile, does Docker rebuild the whole image from scratch or is it able to re-use the intermediate image from right before the relevant RUN command?
为了更好地利用分层，我应该将我的 ARG 声明放在 Dockerfile 的顶部，还是就在使用它们的部分之前?
To better utilize layering, should I place my ARG declarations at the top of the Dockerfile, or just before the section that uses them?
我想我的部分问题是 ARG 指令是否会生成中间层.
I guess part of my question is whether or not an ARG directive generates an intermediate layer.
推荐答案
为了比接受的响应更精确，在 ARG 声明之后，并非所有行都缓存无效.只有那些使用 ARG 值和 RUN 的.docker文档的详细信息:
To be more precise than the accepted response, not all lines are cache invalidated after an ARG declaration. Only those that use ARG values and RUNs. The docker documentation the details:
对构建缓存的影响
ARG 变量不会像 ENV 变量那样持久化到构建的图像中.但是，ARG 变量确实会以类似的方式影响构建缓存.如果 Dockerfile 定义了一个 ARG 变量，其值与以前的构建不同，那么在第一次使用时会发生缓存未命中"，而不是定义.特别是，ARG 指令之后的所有 RUN 指令都隐式使用 ARG 变量(作为环境变量)，因此可能导致缓存未命中.所有预定义的 ARG 变量都免于缓存，除非 Dockerfile 中有匹配的 ARG 语句.
ARG variables are not persisted into the built image as ENV variables are. However, ARG variables do impact the build cache in similar ways. If a Dockerfile defines an ARG variable whose value is different from a previous build, then a "cache miss" occurs upon its first usage, not its definition. In particular, all RUN instructions following an ARG instruction use the ARG variable implicitly (as an environment variable), thus can cause a cache miss. All predefined ARG variables are exempt from caching unless there is a matching ARG statement in the Dockerfile.
- https://docs.docker.com/engine/reference/builder/#impact-on-build-caching
- https://github.com/moby/moby/issues/18017
- https://github.com/moby/moby/pull/18161
- 在这里运行解释:https://github.com/moby/moby/pull/21885
您必须将您的 ARGs 移动到不需要参数的 RUNs 下，以保持层缓存优化.
You'll have to move your ARGs under the RUNs that would not need the argument in order to keep layer cache optimized.
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