如何强制 Docker 重新运行 `apt-get update`？

Question

一些安全更新刚刚发布，我想重建我的 Docker 镜像以利用这些更新。

但是，当我运行docker build .它时，它会立即完成而不更新任何内容，因为Dockerfile. 它甚至不会尝试apt-get update在我的Dockerfile.

apt-get update即使没有任何变化，我如何强制 Docker再次运行命令？

有一个--no-cache选项表示它不会在构建过程中使用缓存，但我希望它之前将缓存用于命令，apt-get update并且我希望将结果保存到缓存中以备下次运行（替换当前缓存的图像），所以我肯定想使用缓存。

我也无法docker rmi删除apt-get运行后生成的图像，因为它拒绝删除该图像作为image has dependent child images.

Answer 1

您可以尝试以下操作：

FROM ubuntu:16.04

# LAYER 1
RUN echo "$(date), layer1" > /tmp/cache.txt

# LAYER 2
RUN echo "$(date), layer2" >> /tmp/cache.txt

# LAYER 3
ARG FORCE_UPDATE=no
RUN echo "$(date), layer3" >> /tmp/cache.txt

# LAYER 4
RUN echo "$(date), layer4" >> /tmp/cache.txt

CMD ["cat", "/tmp/cache.txt"]

第一次构建镜像

$ docker build -t serverfault:900445 .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM ubuntu:16.04
 ---> 0458a4468cbc
Step 2/7 : RUN echo "$(date), layer1" > /tmp/cache.txt
 ---> Running in ac7f6b1e915a
Removing intermediate container ac7f6b1e915a
 ---> 42a6d14cc4cc
Step 3/7 : RUN echo "$(date), layer2" >> /tmp/cache.txt
 ---> Running in ba4cf5b54c35
Removing intermediate container ba4cf5b54c35
 ---> 783957979b21
Step 4/7 : ARG FORCE_UPDATE=no
 ---> Running in 818fd652d5cb
Removing intermediate container 818fd652d5cb
 ---> b8afb473cd9d
Step 5/7 : RUN echo "$(date), layer3" >> /tmp/cache.txt
 ---> Running in 38c0e6cbb94e
Removing intermediate container 38c0e6cbb94e
 ---> 03ac41df5bfa
Step 6/7 : RUN echo "$(date), layer4" >> /tmp/cache.txt
 ---> Running in 0294b5a4078e
Removing intermediate container 0294b5a4078e
 ---> 141667a2d5f3
Step 7/7 : CMD ["cat", "/tmp/cache.txt"]
 ---> Running in 86b852d8222d
Removing intermediate container 86b852d8222d
 ---> dcd57aca0c25
Successfully built dcd57aca0c25
Successfully tagged serverfault:900445

如我们所见，所有层都已构建。再跑一次

$ docker build -t serverfault:900445 .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM ubuntu:16.04
 ---> 0458a4468cbc
Step 2/7 : RUN echo "$(date), layer1" > /tmp/cache.txt
 ---> Using cache
 ---> 42a6d14cc4cc
Step 3/7 : RUN echo "$(date), layer2" >> /tmp/cache.txt
 ---> Using cache
 ---> 783957979b21
Step 4/7 : ARG FORCE_UPDATE=no
 ---> Using cache
 ---> b8afb473cd9d
Step 5/7 : RUN echo "$(date), layer3" >> /tmp/cache.txt
 ---> Using cache
 ---> 03ac41df5bfa
Step 6/7 : RUN echo "$(date), layer4" >> /tmp/cache.txt
 ---> Using cache
 ---> 141667a2d5f3
Step 7/7 : CMD ["cat", "/tmp/cache.txt"]
 ---> Using cache
 ---> dcd57aca0c25
Successfully built dcd57aca0c25
Successfully tagged serverfault:900445

现在所有图层都从缓存中取出。简单检查

$ docker run -it --rm serverfault:900445
Wed Mar  7 15:44:22 UTC 2018, layer1
Wed Mar  7 15:44:23 UTC 2018, layer2
Wed Mar  7 15:44:24 UTC 2018, layer3
Wed Mar  7 15:44:25 UTC 2018, layer4

现在，如果您需要为某些特定图层强制更新缓存，请使用以下命令

$ docker build --build-arg FORCE_UPDATE=$(date '+%s') -t serverfault:900445 .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM ubuntu:16.04
 ---> 0458a4468cbc
Step 2/7 : RUN echo "$(date), layer1" > /tmp/cache.txt
 ---> Using cache
 ---> 42a6d14cc4cc
Step 3/7 : RUN echo "$(date), layer2" >> /tmp/cache.txt
 ---> Using cache
 ---> 783957979b21
Step 4/7 : ARG FORCE_UPDATE=no
 ---> Using cache
 ---> b8afb473cd9d
Step 5/7 : RUN echo "$(date), layer3" >> /tmp/cache.txt
 ---> Running in f8ad1cd195eb
Removing intermediate container f8ad1cd195eb
 ---> b22972691095
Step 6/7 : RUN echo "$(date), layer4" >> /tmp/cache.txt
 ---> Running in 9994175a082e
Removing intermediate container 9994175a082e
 ---> 7ed42904373f
Step 7/7 : CMD ["cat", "/tmp/cache.txt"]
 ---> Running in 67de76e45d43
Removing intermediate container 67de76e45d43
 ---> 833f3faf9fd7
Successfully built 833f3faf9fd7
Successfully tagged serverfault:900445

如您所见，第 1,2 层取自缓存，但第 3 层和所有最新层都已重建

$ docker run -it --rm serverfault:900445
Wed Mar  7 15:44:22 UTC 2018, layer1
Wed Mar  7 15:44:23 UTC 2018, layer2
Wed Mar  7 15:45:35 UTC 2018, layer3
Wed Mar  7 15:45:35 UTC 2018, layer4

再重复一次

$ docker build --build-arg FORCE_UPDATE=$(date '+%s') -t serverfault:900445 .
Sending build context to Docker daemon  2.048kB
Step 1/7 : FROM ubuntu:16.04
 ---> 0458a4468cbc
Step 2/7 : RUN echo "$(date), layer1" > /tmp/cache.txt
 ---> Using cache
 ---> 42a6d14cc4cc
Step 3/7 : RUN echo "$(date), layer2" >> /tmp/cache.txt
 ---> Using cache
 ---> 783957979b21
Step 4/7 : ARG FORCE_UPDATE=no
 ---> Using cache
 ---> b8afb473cd9d
Step 5/7 : RUN echo "$(date), layer3" >> /tmp/cache.txt
 ---> Running in 618880ba45be
Removing intermediate container 618880ba45be
 ---> b0512372ddfd
Step 6/7 : RUN echo "$(date), layer4" >> /tmp/cache.txt
 ---> Running in 0cb552431048
Removing intermediate container 0cb552431048
 ---> 61be6f0c0f21
Step 7/7 : CMD ["cat", "/tmp/cache.txt"]
 ---> Running in 5f9ee850c28e
Removing intermediate container 5f9ee850c28e
 ---> ac73b7754107
Successfully built ac73b7754107
Successfully tagged serverfault:900445


$ docker run -it --rm serverfault:900445
Wed Mar  7 15:44:22 UTC 2018, layer1
Wed Mar  7 15:44:23 UTC 2018, layer2
Wed Mar  7 15:46:10 UTC 2018, layer3
Wed Mar  7 15:46:11 UTC 2018, layer4