Update Makefile to default BINDDIR to nothing if DOCKER_HOST is set

If "DOCKER_HOST" is set, we can usually assume the user is connecting to a remote Docker and thus not bind mount anything by default (meaning the Makefile will more often DWIM for our users).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6886 from tianon/no-binddir-if-docker_host

Update Makefile to default BINDDIR to nothing if DOCKER_HOST is set