clarify new (3.x) singularity ability to get images with @digest specification

[yarikoptic]

See https://github.com/ReproNim/reproman/pull/405#issuecomment-482228428 for initial finding/discussion

[kyleam]

Let's see what comes of https://github.com/sylabs/singularity/issues/3287

[satra]

@kyleam - a few days back i had to authenticate to docker hub to even pull busybox

[kyleam]

@kyleam - a few days back i had to authenticate to docker hub to even pull busybox

Hmm, interesting. I don't think that's what's going on in the snippets from the linked singularity issue, though, because the call succeeds despite the warning if I drop the @ID bit:

$ singularity pull shub://vsoch/hello-world      
WARNING: Authentication token file not found : Only pulls of public images will succeed
 62.32 MiB / 62.32 MiB [=========================================] 100.00% 8.31 MiB/s 7s