docker build this:
docker build . -t gitlab/gitlab-emma
run it with ./start-gitlab
access GitLab at http://localhost:8029
This crucial bit is, the plugin directory is on my local drive so I can mess with it.
like, move ./save-to-file.rb
(a sad modification of GitLab's sad example) to $HOME/install/gitlab/plugins and then it will write a file (overwriting it each time, this is the sad part) to $HOME/install/gitlab/write-crap-here.
I have these directories in $HOME/install/gitlab : config data logs plugins write-crap-here