Pipeline step "Build and conditionally push image to ghcr.io" fails because of invalid tag in some instances

[Dreistein]

The pipeline step Build and conditionally push image to ghcr.io in the publish.yml fails when the source branch name contains invalid characters. E.g.
ERROR: invalid tag "ghcr.io/austriandatalab/baas-backend:feature/typst-doc-creation": invalid reference format

Allowed characters are A-Z, a-z, 0-9, _, -, .
(https://docs.docker.com/reference/cli/docker/image/tag/)

We should replace all invalid characters with the empty string. Thoughts?

[Dreistein]

For reference: this affects #26