Update docker build instructions

Docker building still won't work until I look at node-creator, but
fix the build instructions. "docker build - <..." works in a temp
directory which doesn't know where the script is. Needs to be
symlinked into the local directory instead.

Also, docker build no longer directly runs the entrypoint, so it
should be tagged and run independently

Change-Id: I6ea0ea442f4a11fc9edc7940d2a8b1906511543f
Signed-off-by: Ryan Barry <rbarry@redhat.com>