Docker multi stage builds
requested to merge MichaelPereira/gitlab-runner-docker-cleanup:docker-multi-stage-builds into master
As stated in #5, it's ironic that a cleaning tool takes so much space itself (947MB). Taking advantage of docker multi-stage builds, the tool now only takes 14.3MB.
This change might also fix a lot of #14 since it will be just the binary running on top of the latest alpine.