bashbrew: account for namespaces when sorting repos

[7AC]

When sorting repos sort.go looks at the FROM lines in the Dockerfiles. If --namespace is set it needs to be taken into account as the line will be in the form FROM <namespace>/<image>[:<tag>]. This also adds --namespace to bashbrew list as it also sorts.

[tianon]

Heh, I debated doing this exact thing back and forth a lot when I originally wrote bashbrew list (hence why there's a namespace := "" in there as a placeholder of sorts).

The amount of places we pass this around has me wondering if maybe it's time to instead move --namespace to be a global option on par with --arch so we don't have to pass it around so much (and because it needs to affect basically everything anyhow). The only real exception is bashbrew tag, which needs to take the non-namespaced images and tag them into a different namespace, so it would likely just adjust to have a --target-namespace parameter or something similar.

I need to noodle on this a little more. 👍 ❤️

[tianon]

I gave it more thought and took a stab at porting your implementation (especially now that #5929 is finally resolved). 👍

[7AC]

Thanks @tianon ! Anything else I need to do to merge?

[tianon]

I'd just like Joe to take a look as well, which he'll hopefully do today 👍