Verify ~/.docker/config.json credentials usage

[dtrudg]

The apptainer issue...

apptainer/apptainer#1616

.. was opened because a user noticed a loss of fall-back to using registry credentials in ~/.docker/config.json (which are stored by docker login).

We should investigate whether we are advertising this behaviour in our documentation still, and whether it works or not. We need to decide what is correct - fall back or not.

Note that our code in this area is quite different to that which Apptainer modified in their PR to fix the issue. We don't use oras-go any more.