Dockerfile: Upgrade to Ubuntu 18.04 bionic; general cleanup.
Dockerfile: Upgrade to Ubuntu 18.04 bionic; general cleanup.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

Galexrt/docker-zulip,Galexrt/docker-zulip,Galexrt/docker-zulip