Removed cross-compilation bobbins

Cross-compilation isn't working, and it's misleading to suggest it
does by having these here.
Changed payload format

We need the docker (host network) IP, and the FQDN rather than just the domain.
Merge remote-tracking branch 'bb/master'
Added container updates watcher
Simplified args and added minimal event handling
Put both image builds in docker/Makefile
Moved updater into server

While we're only reacting to containers going away, it's more
convenient to bundle the docker events watcher into the server.

I've added a flag to switch it on; and switched it on by default in
the container.

I also implemented `Zone.DeleteRecordsFor(ident)`, which removes all
records for a particular container.

One minor change to the structure of the code is that the Zone is now
constructed *outside* the server, and given to it and the updater.
Print out command-line args, for debugging