Docker containers don't get removed after execution

Summary

When using the docker executor and running a build, the "build" and "predefined" containers will stay after the build.

Expected behavior

I expect the containers to be removed afterwards when the executor cleans up.

Used GitLab Runner version

Version:      9.1.0
Git revision: 0118d89
Git branch:   9-1-stable
GO version:   go1.7.5
Built:        Fri, 21 Apr 2017 14:32:04 +0000
OS/Arch:      linux/amd64

I looked a bit into this and it seems like if all created containers used to go into the array builds of the executor struct (executor_docker.go). The method Cleanup then took care of removing them afterwards. But since 38dff79d9a36df5e0f7df0c52654aabc1ab452f8 builds stays completly empty because the line s.builds = append(s.builds, container) in createContainer got removed.