Build and release on each commit to fdroid/server

Right now new image is released only when a commit is added to this repo. The usual solution would be to have the Dockerfile in the same repo as code (and changing git clone to ADD), so it's built every time the source changes. If you want to keep it in a separate repo, it'd be good to have gitlab run the pipeline on each commit to fdroid/server if possible, but I'm not very familiar with gitlab pipelines.