docker在git checkout同一文件时添加缓存

Question

我需要重用docker缓存在CI服务器构建映像中使用checkout项目。

签出相同文件时，Docker ADD不使用缓存。

我在git分支 A中docker build -t somename .通常使用docker缓存执行它，但是如果我通过转到分支
Bgit checkout B，不执行任何操作，通过转到分支 Agit checkout A并再次运行，docker build -t somename .则仅在第一个ADD之前使用docker缓存。

这里的例子：

Docker文件

# DOCKER-VERSION 0.10.0
FROM myregistry:5000/ruby:2.1.2-1
MAINTAINER ME

# Gem sources
RUN gem source -r https://rubygems.org/
RUN gem source -a http://gems.mydomain

# Never install a ruby gem docs
RUN echo "gem: --no-rdoc --no-ri" >> ~/.gemrc

# gems install
RUN mkdir /foo
WORKDIR /foo

RUN gem install bundler
ADD Gemfile /foo/Gemfile
RUN bundle install

# expose ports
EXPOSE 8080

从构建日志

Sending build context to Docker daemon 19.54 MB
Sending build context to Docker daemon 
Step 0 : FROM myregistry:5000/ruby:2.1.2-1
 ---> 9ce683a713b4
Step 1 : MAINTAINER ME
 ---> Using cache
 ---> 8f54114fd2e7
Step 2 : RUN gem source -r https://rubygems.org/
 ---> Using cache
 ---> f58a08708863
Step 3 : RUN gem source -a http://gems.mydomain
 ---> Using cache
 ---> 3e69e17c5954
Step 4 : RUN echo "gem: --no-rdoc --no-ri" >> ~/.gemrc
 ---> Using cache
 ---> 1edb37962dd4
Step 5 : RUN mkdir /foo
 ---> Running in 3d3441c34ee3
 ---> aeb90f00bc9b
Removing intermediate container 3d3441c34ee3
Step 6 : WORKDIR /foo
 ---> Running in 84b881d8f621
 ---> 10e1d8984458
Removing intermediate container 84b881d8f621
Step 7 : RUN gem install bundler
 ---> Running in 31e98523ce46
Successfully installed bundler-1.6.2
1 gem installed
 ---> 84d5195ab831
Removing intermediate container 31e98523ce46
Step 8 : ADD Gemfile /foo/Gemfile
 ---> 3e5f2675ee22
Removing intermediate container c90e8be5ea17
Step 9 : RUN bundle install
 ---> Running in ac0e83e5eebb    
Fetching gem metadata from http://gems.mydomain/......
Fetching additional metadata from http://gems.mydomain/..
Resolving dependencies...
Installing rake 10.3.2
Installing i18n 0.6.9
Installing multi_json 1.10.1
.
.
.
Installing railties 3.2.19
Installing responders 0.9.3
Using bundler 1.6.2
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.    
 ---> 19beae703adb
Removing intermediate container ac0e83e5eebb
Step 10 : EXPOSE 8080
 ---> Running in 1b1e55d349e5
 ---> 32405bdac6d1
Removing intermediate container 1b1e55d349e5
Successfully built 32405bdac6d1

git checkout B

git checkout A

docker build -t somename。

第二个构建日志

Sending build context to Docker daemon 19.52 MB
Sending build context to Docker daemon 
Step 0 : FROM myregistry:5000/ruby:2.1.2-1
 ---> 9ce683a713b4
Step 1 : MAINTAINER ME
 ---> Using cache
 ---> 8f54114fd2e7
Step 2 : RUN gem source -r https://rubygems.org/
 ---> Using cache
 ---> f58a08708863
Step 3 : RUN gem source -a http://gems.mydomain
 ---> Using cache
 ---> 3e69e17c5954
Step 4 : RUN echo "gem: --no-rdoc --no-ri" >> ~/.gemrc
 ---> Using cache
 ---> 1edb37962dd4
Step 5 : RUN mkdir /foo
 ---> Using cache
 ---> aeb90f00bc9b
Step 6 : WORKDIR /foo
 ---> Using cache
 ---> 10e1d8984458
Step 7 : RUN gem install bundler
 ---> Using cache
 ---> 84d5195ab831
Step 8 : ADD Gemfile /foo/Gemfile
 ---> 4977e35c80f7
Removing intermediate container bd59cc0d5e51
Step 9 : RUN bundle install
 ---> Running in 6ff16f32e94a
Fetching gem metadata from http://gems.mydomain/......
Fetching additional metadata from http://gems.mydomain/..
Resolving dependencies...
Installing rake 10.3.2
Installing i18n 0.6.9
Installing multi_json 1.10.1
.
.
.    
Installing railties 3.2.19
Installing responders 0.9.3
Using bundler 1.6.2
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
 ---> d9332f9035c3
Removing intermediate container 6ff16f32e94a
Step 10 : EXPOSE 8080
 ---> Running in b20252a00160
 ---> 4d9932882e06
Removing intermediate container b20252a00160
Successfully built 4d9932882e06

Answer 1

当文件的mtime值更改并且git不跟踪文件的mtime值时，Docker使docker
build缓存无效。这种缓存失效也会在其他情况下出现，例如在持续集成或涉及docker，git和branchs的构建环境中。

在要求docker构建容器之前，我一直在运行的Makefile中使用“触摸”目标：

touch:
    @echo "Reset timestamps on git working directory files..."
    find ./ | grep -v .git | xargs touch -t 200001010000.00

接下来，始终在同一Makefile中运行make touch，docker build或始终在同一基于docker的“ build”目标中运行…

另一个选项是设置一个git钩子，该钩子会自动修改mtime值：https
:
//git.wiki.kernel.org/index.php/ExampleScripts#Setting_the_timestamps_of_the_files_to_the_commit_timestamp_of_the_commit_which_last_touched_them

另一个可能的解决方案是派生docker并将mtime从其缓存定义中删除：https
:
//github.com/docker/docker/blob/master/pkg/tarsum/tarsum.go

注意 ：从docker
1.8开始，mtime使缓存无效时不再考虑在内。拉取请求＃12031更新了此行为