Docker builds for dev

[silvanheller]

Currently, PRs using the dev branch of Cottontail (e.g. vitrivr/cineast#285) cannot be tested with the CI if there are changes to the Cottontail API / underlying functionality which is only available on dev. One thing that we could do is automatically push a version-dev container whenever dev is updated. As it's not a major pain point, this has low priority and we can also leave it "as-is".
Any thoughts on that matter?