GUACAMOLE-407: Update Docker image to use more recent packages by switching to Ubuntu.
GUACAMOLE-407: Update Docker image to use more recent packages by switching to Ubuntu.

apache/guacamole-server,mike-jumper/incubator-guacamole-server,glyptodon/guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,glyptodon/guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server