Add release candidates

[joshtrichards]

• To encourage wider use of new Server releases
• To ease use of RCs by micro-services Docker users
• To get feedback before final releases are published

This is encouraged per Docker Official documentation.

A variation, rather than publishing via the Official Images path, would be publish these intermediate images via the GitHub Container Registry maybe? https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry

[isdnfan]

In the past it was decided not to provide beta images #1731 because of many changes and issues.
RC is expected to be more stable and earlier testing would improve the micro-services image a lot.

[joshtrichards]

Related previous PR: #2012