Duplicating my answer here to make it easier to find.
The repo in question is: https://bitbucket.org/ueacomputervision/image-labelling-tool/src/master/
and when you fill in the form it looks like this:
You need to explicitly specify the revision you want, you can’t write
master for the “arbitrary git URL” mode as we have no way to resolve that “word” into a revision, which is what we need to check the build cache. For GitHub and GitLab there is a little bit of magic to talk to the respective APIs to resolve it first. Could add this for bitbucket as well if someone is interested enough to open a PR.