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.