ggcr: unable to reuse authenticated transport with v1/remote

[benmoss]

Describe the bug

I'd like to initialize an http.RoundTripper after doing an auth handshake and then use it to perform several requests to a repository. Right now the v1/remote functions all create a new http.RoundTripper via transport.NewWithContext, even if you pass one to the method via the option.WithTransport call.

To Reproduce

I made this sample app to demonstrate the problem: https://gist.github.com/benmoss/620ec0b372be90c2471fdff3d80564fb

With the debug logs enabled you can see several calls to GET https://gcr.io/v2/, which should be unnecessary since we already know it allows anonymous access.

Expected behavior

It should skip the extraneous /v2 pings

Additional context

I was thinking maybe it would make sense for registry to do some type check on the transport, similar to how we do a check for a transport.Wrapper right now to skip some wrapping, but in this case for authentication. Maybe the user can just pass a transport.Wrapper and that will be used? I'm not sure why not other than its a change in the contract.

Support for using the same transport to talk to multiple registries would also get more complicated, but this is similar to remote.MultiWrite