Support forked repos

[edtan]

This MR addresses #26 by checking the GitLab merge request's source_project_id and cloning from that project instead of the source config's uri.

[eperdeme]

I guess this projects stale as does not seem much activity in a long time from the author. Does anybody have an active fork or moved to an alternative resource ?

[edtan]

I've merged these changes into the master branch of my fork of this repo, and the corresponding Docker image is available on Docker Hub (under edtan1). However, I haven't used this resource in a while.