Maintenance: Generate docker tags for Zammad major.minor and major versions

[mgruner]

This change will

• add major.minor and major docker tags that can be used to get automatic updates with docker-compose and helm
• remove the dysfunctional semver tag configuration - does not work because we don't use semver tags with v1.2.3
• stop creating the duplicated zammad-* tags

[mgruner]

@monotek can you please have a look? With Zammad 6.2, I would like to start pushing and updating tags like 6.2 and 6 (like it is done for https://hub.docker.com/r/selenium/standalone-chrome/tags, for example).
This will allow users to receive patchlevel updates automatically from docker-compose or helm without having to update the .env all the time.

The semver config we had there didn't seem to work at all, and I would also like to remove the zammad-* tags if you agree. What do you think?

[monotek]

lgtm but ci fails 🤷

[mgruner]

Thanks @monotek. CI was improved in the meantime and should work now. I will then remove the debug comments and merge this.