Push to dockerhub on successful builds.

If master builds successfully then push the results
to dockerhub.

We assume that the dockerhub username and password will
be in the environment. This will use travis env vars so
that the secrets aren't available to forks.

The on: branch: master means that other branches wouldn't
push to dockerhub.

.travis.yml

@@ -6,3 +6,9 @@ services:
 before_install:
   - sudo apt-get -qq update
   - sudo apt-get install -y debian-archive-keyring debootstrap
+deploy:
+  provider: script
+  script: pushall
+  skip_cleanup: true
+  on:
+    branch: master

pushall

@@ -6,6 +6,10 @@ unstable
 
 BASENAME=jamesw/minideb
 
+if [ -n "$DOCKER_PASSWORD" ]; then
+    docker login -u $DOCKER_USERNAME -p $DOCKER_PASSWORD
+fi
+
 for DIST in $DISTS; do
     docker push $BASENAME:$DIST
 done