Description
Is your feature request related to a problem? Please describe.
Sometimes it is necessary to use time-limited, or specific-purpose, credentials to push/pull to/from OCI registries.
SingularityCE does support using environment variables, but sometimes a file storing credentials is more convenient.
Other runtimes have an --authfile flag that allows a specific file to be used as the store of OCI credential, rather than the default. We should provide this functionality in SingularityCE.
The --authfile flag would need to be available on the singularity registry command, and any other commands that consume credentials.
Some thought needs to be given to the way in which we currently store OCI login information split between remote.yml and the credentials file (which is in docker config format). How should this split be reconciled with an --authfile flag?