Fix GitLab external URL configuration

[timofurrer]

This will fix the acceptance testing instance to have a proper external
URL configured. With that redirects within GitLab will work (e.g. from
the WebIDE back to the repository) and stuff like clone URLs etc.

[timofurrer]

This change seems to have the drawback that GitLab cannot reach a repository it hosts with the external URL. We are currently having this project in the TestAccGitlabProject_importURL which tries to import a repository hosted on the same GitLab instance.

Error: POST http://127.0.0.1:8080/api/v4/projects: 422 {message: http://127.0.0.1:8080/root/base-137441643613142598.git is not a valid HTTP Git repository}

One possible solution might be to change the nginx port to the same as the port used for the container port forwarding. Let's see ...

[timofurrer]

I'm currently having some issues with getting this ready - since this is not urgent - I'm going to close for now and will re-open at a later time ;)