dockerdeb: add Ubuntu Trusty

One problem reported by lintian is "bad-distribution-in-changes-file unstable"
in changelog, but the current changelog for the official package in Ubuntu also
uses that distribution name (unstable), because they import from Debian. This
certainly doesn't stop the build process.

Makefile

@@ -168,6 +168,10 b' docker-debian-jessie:'
 	mkdir -p packages/debian-jessie
 	contrib/dockerdeb debian jessie
 
+docker-ubuntu-trusty:
+	mkdir -p packages/ubuntu-trusty
+	contrib/dockerdeb ubuntu trusty
+
 fedora20:
 	mkdir -p packages/fedora20
 	contrib/buildrpm

contrib/docker/ubuntu-trusty

@@ -1,4 +1,4 b''
-FROM debian:jessie
+FROM ubuntu:trusty
 RUN apt-get update && apt-get install -y \
   build-essential \
   debhelper \