Tavis-ci to upload images to the Docker hub, after succesfull tests.

For this to work, the following variables will need to be set at Docker hub:
 - DOCKER_ORG	first part of docker repo name (example: mailu)
 - DOCKER_UN	Docker-hub login username (private)
 - DOCKER_PW	Docker-hub login password (private)
Note that also Docker hub autobuilds will have to be disabled.

tests/deploy.sh

@@ -0,0 +1,4 @@
+#!/bin/bash
+
+docker login -u $DOCKER_UN -p $DOCKER_PW
+docker-compose -f tests/build.yml push