[Standardization] Registry Standard from DR SCS-0212

[cah-hbaum]

Currently, SCS doesn't have a standard for the registry, which should be used to control images for a cluster. At the moment, only a Decision Record is available ("scs-0212-v1-requirements-for-container-registry"), which should be used to create the standard.

Definition of Done:

• [x] The standard document is written.
• [x] The document is reviewed and approved.
• [x] A follow-up issue for tests is created. -> https://github.com/SovereignCloudStack/issues/issues/662

[cah-hbaum]

After talking with @matofeder, we decided that it makes sense to split scs-0212-v1-requirements-for-container-registries.md into a standard and a decision record.
The standard should contain a vendor-agnostic set of must-haves and/or nice-to-haves. Depending on this standard, a Decision Record should be created, that contains the decision to use Harbor for the SCS reference implementation, since it meets all requirements for the SCS standard.

Edit: I'm gonna create the DR and standard after the discussion in 352 is resolved, since the template and location of DRs could possibly be changed in the future.