[Docker Hub] Support more granular access tokens

[sudo-bmitch]

Tell us about your request
Access tokens that grant limited access to either read only, or a subset of repositories, would be useful for CI pipelines. This would allow a CI job to authenticate for higher rate limits without unnecessary access. This would also allow a CI job to be limited to only the repository they are updating.

Which service(s) is this request for?
Docker Hub

Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?
Tokens for Docker Hub give push access to everything the user has access to today, which often exceeds the needed access.

Are you currently working around the issue?
Currently I'm granting excessive access. Potentially I could create a separate account and configure the user as a team member for a specific repository.

Additional context

[dduportal]

This feature is available as per #115 but requires a non-free plan.