Fix DCO parsing for numeric github handles like cpuguy83

Turns out, "alphanumeric" actually means both "alpha" AND "numeric". Dur.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6883 from tianon/fix-dco-parsing-for-numeric

Fix DCO parsing for numeric github handles like cpuguy83
Update Travis to use pretty Docker-based workers

These start up much faster and the only caveat is that we can't use "sudo" (which we don't currently use anyhow).

Also, I've updated the Go version here to match what's in the Dockerfile.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6884 from tianon/use-docker-based-travis

Update Travis to use pretty Docker-based workers
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)