make: turn ubuntu docker into template

This allows us to easily add more ubuntu docker targets (which following
patches will do).

Also, we no longer need the mkdir command.

contrib/docker/ubuntu.template

@@ -0,0 +1,11 b''
+FROM ubuntu:__CODENAME__
+RUN apt-get update && apt-get install -y \
+  build-essential \
+  debhelper \
+  dh-python \
+  devscripts \
+  python \
+  python-all-dev \
+  python-docutils \
+  zip \
+  unzip

.hgignore

@@ -30,6 +30,7 b' build'
 contrib/chg/chg
 contrib/hgsh/hgsh
 contrib/vagrant/.vagrant
+contrib/docker/ubuntu-*
 dist
 packages
 doc/common.txt

Makefile

@@ -181,8 +181,10 b' docker-debian-jessie:'
 	mkdir -p packages/debian-jessie
 	contrib/dockerdeb debian jessie
 
-docker-ubuntu-trusty:
-	mkdir -p packages/ubuntu-trusty
+contrib/docker/ubuntu-%: contrib/docker/ubuntu.template
+	sed "s/__CODENAME__/$*/" $< > $@
+
+docker-ubuntu-trusty: contrib/docker/ubuntu-trusty
 	contrib/dockerdeb ubuntu trusty
 
 fedora20: