Dockerfile 未执行第二阶段

Question

我从 Docker 那里得到了一些奇怪的行为，我找不到任何地方提到它。它似乎是随机跳过阶段，即使是我简单地从网上论坛复制粘贴的多阶段 Dockerfile。

我的 dockerfile 是：

FROM alpine as base
RUN echo "1"

# SKIPPED
FROM base as mid
RUN echo "2"

FROM base as final
RUN echo "3"

输出：

docker build -t test .\ --no-cache
[+] Building 2.0s (7/7) FINISHED
 => [internal] load .dockerignore                                                                                  0.0s
 => => transferring context: 34B                                                                                   0.0s
 => [internal] load build definition from Dockerfile                                                               0.0s
 => => transferring dockerfile: 143B                                                                               0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                                   1.3s
 => CACHED [base 1/2] FROM docker.io/library/alpine@sha256:...  0.0s
 => [base 2/2] RUN echo "1"                                                                                        0.3s
 => [final 1/1] RUN echo "3"                                                                                       0.4s
 => exporting to image                                                                                             0.0s
 => => exporting layers                                                                                            0.0s
 => => writing image sha256:...                      0.0s
 => => naming to docker.io/library/...                                                                 0.0s

这是什么原因造成的？

Answer 1

Buildkit 使用依赖图。它查看目标阶段，默认情况下是最后一个阶段：

FROM base as final
RUN echo "3"

从那里它看到base需要构建这个阶段，所以它拉入基础阶段：

FROM alpine as base
RUN echo "1"

从那里就完成了，不需要构建中间阶段来创建您的目标图像。FROM或 aCOPY --from中没有需要它的依赖项。这种行为不同于经典的 docker build，后者按顺序执行步骤直到达到目标阶段，这也是 buildkit 速度更快的原因之一。