Adding ACR as a remote (docs)

[radoye]

Version of Singularity:

3.2.0-1

Expected behavior

An example of how to properly add an ACR as a remote?

singularity remote add devhub <my registry>.azurecr.io
singularity remote login devhub 
singularity push xyz.sif devhub

Actual behavior

It's unclear to me what should go into the remote command. Simply putting the Azure supplied "login server" string (.azurecr.io) in does not make Singularity client happy.

Steps to reproduce behavior

singularity push dev.sif oras://<my registry>.azurecr.io/dev:0.1.1
FATAL:   Couldn't push image to library: no authentication token, log in with 'singularity remote` commands

[dtrudg]

The remote command is exclusively for Sylabs cloud services, i.e. the container library library:// URLs, remote builder, key store.

ORAS support was only added in 3.3.0 - the older 3.2.0 here is incorrectly trying to interpret the oras:// URI as if it was a library one.

We do need some oras docs.