We try to emulate the standard Docker CLI's image handling.  Thus, if
user specifies an image like 'ubuntu', we turn that into
'ubuntu:latest'.  If user does not supply a registry host, we try
'registry.hub.docker.com'.  If they do not specify a registry host or
specify a registry host that is either registry-1.docker.io or
registry.hub.docker.com, and their image does not contain a /, we
prepend 'library/' to it (I *think* this is the right heuristic, but
it's inference).

For Dockerfiles, we must be able to download it, and it must contain a
line matching ^\s*FROM (i.e. a valid FROM instruction, which all
Dockerfiles must have).  We try to support DOS-mode textfiles too, but
only ASCII.

Might need to loosen these checks.