Explore the possibility of merging DockerHub and GAR for docker actions

[dsotirakis]

These actions seem to be sharing a lot. Let's explore the idea of creating one action to rule them all. Things to consider:

• The authentication will be different between the two cases.
• There must be a step to find and resolve the actual registry - whether it's DockerHub, GCR, GAR.
• ...

[iainlane]

If we fix #96, would this not be needed? When users are pinned, changing the default won't break things.

[dsotirakis]

Ah this issue is more about merging the login-to-gar and login-to-dockerhub actions together. So for example if you want to authenticate in a docker registry, it can resolve to either docker.io/... or us-pkg.docker.dev/....

[iainlane]

Ah this issue is more about merging the login-to-gar and login-to-dockerhub actions together. So for example if you want to authenticate in a docker registry, it can resolve to either docker.io/... or us-pkg.docker.dev/....

lol wait a second, I meant to write that comment on #91