Feature: Add DockerImage classes for docker.io, nvcr.io, and quay.io

[georgiastuart]

Corresponds with singularityhub/singularity-hpc#697 . Resolves #480 and #481.

[vsoch]

This is amazing! You are two steps ahead! 🙌

Do you know why the digests are sometimes taking up two lines? I think likely we would want the newline removed - I am not sure what kind of funny business it will cause.

[georgiastuart]

@vsoch I figured out the two line issue: when a line goes over 80 chars, the digest gets put on the next line. It doesn't appear to cause problems (it's been present for a while), but increasing line length to 120 solves this in all but extreme cases: singularityhub/singularity-hpc@f67c7a8

120 feels pretty wide, but since these files aren't meant to be hand-edited a whole lot it's probably OK. Thoughts? 100 would likely get most tags but won't have as much breathing room.

[vsoch]

I would say set it to a very large value to prevent line wrapping, e.g., 4096. Is there any drawback?

[georgiastuart]

The main drawback is the line length is universal for the written file. So if there's a long description or something, it won't wrap. If that's not an issue, I'll set it to 4096!