(apro) docker.mk: Be a little more robust
(apro) docker.mk: Be a little more robust

 - Correctly handle $(VERSION) changing in the middle of a run
   * accomplish this by pulling the saved tag name from the .docker file,
     rather than re-computing it
   * assume "latest" is not a value that VERSION could ever take
   * exception: In the %.docker.push rule, I don't handle it changing
     between the `tag` and `push` lines.  That would be a pain, and they
     happen very fast.
 - Always quote the tag name
 - Fix xargs usage on macOS
 - Don't leave .tmp.%.docker.tmp files around
 - Don't re-tag :latest if the image didn't change
 - Don't assume VERSION is set (some places did, some places didn't)
 - Don't include version.mk
 - Don't allow starting a push with the wrong port-forward, block until we
   have the lock.

datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador,datawire/ambassador