Inherit registry options as defaults for undefined mirrors

[rjsocha]

Reference: #6177, https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md

This PR adds support to inherit registry options as defaults for mirrors without explicit config.

Before:

[registry."docker.io"]
  mirrors = ["mirror1.example.dev", "mirror2.example.dev"]

[registry."mirror1.example.dev"]
  http = true

[registry."mirror2.example.dev"]
  http = true

After:

[registry."docker.io"]
  mirrors = ["mirror.example.dev", "mirror2.example.dev"]
  http = true

[tonistiigi]

I don't think we should do it. Better to leave mirrors secure by default and users can always reconfigure them if needed. If you like, you can update the docs to make it clearer that mirrors require their own custom configuration.

[rjsocha]

Thanks for the clarification. A note in the docs would definitely help others configure this correctly. Feel free to close this PR.