Support docker credentials in offsetgen

[MrAlias]

Is your feature request related to a problem? Please describe.

When a developer runs a full offset update, or tries to add a new offset, on a new system without a cache of all the docker images of Go Dockerhub will rate-limit their requests for containers. This is something like 100 pulls a day, but more requests are needed for a completely new setup.

Describe the solution you'd like

Support user authentication in offsetgen. If it is set in the CLI or via the environment use that auth to pull images so the rate limit is not a blocker.

Describe alternatives you've considered

Currently you can work around this by authenticating with the docker CLI and just manually pulling all the images you need.

Additional context

https://docs.docker.com/docker-hub/download-rate-limit/

[damemi]

I'm hitting this now so just wanted to share what I've found: I think the docker functions like ImagePull have an Options struct that takes encoded registry auth credentials. Seems like we need to get the docker auth credentials locally at startup, base64 encode them, and pass them to these functions