hghave: add a check for docker support
This currently refuses to operate if on a non-Linux host. I suspect
that Docker running on FreeBSD 11 or on an Illumos derivative would
work fine, but I don't have ready access to such a system.
On OS X using boot2docker (I used a hacky xhyve-based one for
testing), it won't work because $TESTTEMP doesn't end up inside the
set of directories that get forwarded to the boot2docker VM, so you
can't actually drop debs in the $TESTTEMP at all. It would be possible
(probably even trivial) to hack around this by using a randomly-named
temporary directory inside the working directory, but that seems
unlikely to be useful enough to justify the ugliness.