According to env-ci, a slug takes the form owner_name/repo_name of the repository currently being built. It's currently implemented using BITBUCKET_REPO_SLUG.

However, according to Bitbucket, BITBUCKET_REPO_SLUG is the URL-friendly version of a repository name. They provide foobar and foo-bar as examples. Clearly this is incompatible.

I think this should be changed to use BITBUCKET_REPO_FULL_NAME, which is described as The full name of the repository (everything that comes after http://bitbucket.org/). While that's poorly described, it appears to be exactly what we're after (owner_name/repo_name), as confirmed by some googling.