add docker- prefix to generated hostnames

this ensures rfc-compliant hostnames by ensuring that are (a) not all
numeric and (b) start with an alpha.

Given a command like:

    docker run -it fedora /bin/bash

This will result in something like:

    bash-4.2# hostname
    docker-6d41d2951bd7

this addresses docker issue #7756

Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>