Add support to explicitly providing registry credentials

[migmartri]

Currently, the relok8s CLI loads credentials from the local Docker keychain (~/.docker/config.json). Relok8s as a library already supports passing credentials explicitly #70 so the next steps would be to expose that functionality to the CLI.

One possible implementation could be accepting env variables similar to what charts-syncer does, flags or config files could be an alternative too. If we go to that route it might make sense to look into if https://github.com/spf13/viper could be a good fit or an overkill.

cc/ @josvazg