Dockerfile 中 ARG 的使用与其作用域(scope)探究

## 在第一个 FROM 之前的所有 ARG , 在所有 FROM 中生效, 仅在 FROM 中生效
ARG image

FROM $image as stage1
RUN echo "stage1 -> base from image is : $image "
    # result: stage1 -> base from image is :

FROM $image as stage11
RUN echo "stage11 -> base from image is : $image "
    # result: stage11 -> base from image is :

FROM alpine:3.12 as stage2
## 在 FROM 后的 ARG, 仅在当前 FROM 作用域生效。即尽在当前 阶段 (stage) 生效
ARG image
RUN echo "stage2 -> base from image is : $image "
    # stage2 -> base from image is : alpine:3.12

FROM alpine:3.12 as stage21
RUN echo "stage21 -> base from image is : $image "
    # stage21 -> base from image is :

# docker build --build-arg image=alpine:3.12 --no-cache .

Sending build context to Docker daemon  3.072kB
Step 1/10 : ARG image
Step 2/10 : FROM $image as stage1
 ---> d6e46aa2470d
Step 3/10 : RUN echo "stage1 -> base from image is : $image "
 ---> Running in ecb7be5dd9cc
stage1 -> base from image is :  ### image 结果未输出
Removing intermediate container ecb7be5dd9cc
 ---> 04807c8d53be
Step 4/10 : FROM $image as stage11
 ---> d6e46aa2470d
Step 5/10 : RUN echo "stage11 -> base from image is : $image "
 ---> Running in a90e45076345
stage11 -> base from image is :       ### image 结果未输出
Removing intermediate container a90e45076345
 ---> f2dbce837a1b
Step 6/10 : FROM alpine:3.12 as stage2
 ---> d6e46aa2470d
Step 7/10 : ARG image
 ---> Running in 5c8cec4c2f22
Removing intermediate container 5c8cec4c2f22
 ---> 999d9990bd91
Step 8/10 : RUN echo "stage2 -> base from image is : $image "
 ---> Running in 4407dcb0e0bb
stage2 -> base from image is : alpine:3.12     ### image 结果输出
Removing intermediate container 4407dcb0e0bb
 ---> e5ddd7a84f81
Step 9/10 : FROM alpine:3.12 as stage21
 ---> d6e46aa2470d
Step 10/10 : RUN echo "stage21 -> base from image is : $image "
 ---> Running in 64a0a3bb090c
stage21 -> base from image is :      ### image 结果未输出
Removing intermediate container 64a0a3bb090c
 ---> 82665f9a1037
Successfully built 82665f9a1037