Add ability to log into a registry on a per-app basis

[josegonzalez]

Description of feature

At the moment, registry login is global, but it may be desirable to have different credentials for different apps on the same registry.

We can do this by specifying a --config flag internally when calling docker login via registry:login. This would point to a new docker config file that has the app name in it. We would also expose a new --global flag that would replicate the existing behavior - global login - or an app could be specified as the first argument.

To ease the transition, we could try forcing --global if there are only 3 args instead of 4 to registry:login, displaying a warning in those cases.

Example SO link here on the --config flag: https://stackoverflow.com/questions/50177884/how-to-use-multiple-auths-logins-for-same-docker-registry/55635346#55635346