Fix remaining v2 -> v3 changes in docs and configuration

[thaJeztah]

Description

I noticed some things that we need to address before v3.0.0 w.r.t. configuration or the official image, and documentation; this is just a quick list, but there may be more;

• update the documentation to refer to the new image tag (documentation currently uses registry:2)
• update the location of the configuration file (/etc/docker/registry/config.yaml)
  • the path should likely no longer contain "docker"; decide on a new location
  • can we have some / do we need some fallback for the old location? (symlink, or ideally; binary checking both?)
  • update the official image accordingly; https://github.com/distribution/distribution-library-image/blob/73483231b14174569cbc2ecaf8af15a990f460c6/Dockerfile#L32
  • is there a default location that the registry uses? if so, can we remove the ENTRYPOINT and let the registry use the default location (if present)?
• verify the default configuration file in the official image. The registry-library-image has a default / example configuration file; https://github.com/distribution/distribution-library-image/blob/eb58390c830bc5017f1a4eafc365b45206ad21fe/config-example.yml. We need to;
  • verify if the example is still correct for v3 (is version 0.1 still accurate?)
  • do we have a canonical "default" config file in the main repository that we can use instead?
  • or; does the registry have an option to output the default config (similar to containerd's containerd config default, which dumps the default config file)?
  • are any of the options specified different from built-in defaults?
  • if none are different from the default, can we remove the configuration file entirely?

[thaJeztah]

cc @milosgajdos @dvdksn @Jamstah

[milosgajdos]

🫠 I wonder if we should create v3-stable Mileston in GH to track there

[Jamstah]

🫠 I wonder if we should create v3-stable Mileston in GH to track there

I think we can just reuse the 3.0.0 milestone