Automatic creation of review images

- Enable bors-ng for better merging
- Re-enable mergify with a fixed up syntax, it will now only issue bors commands.
- Pull request build images are now pushed to docker hub
- Automatic backport rule
- Fix build tag for branches with slash (/)
- Only push when master and 1.6 target branch

tests/deploy.sh

@@ -1,4 +1,16 @@
 #!/bin/bash
 
-docker login -u $DOCKER_UN -p $DOCKER_PW
-docker-compose -f tests/build.yml push
+# Re-tag images for PR testing
+if [ $TRAVIS_PULL_REQUEST != false ]; then
+    export MAILU_VERSION="${TRAVIS_BRANCH}-${TRAVIS_PULL_REQUEST}"
+    export DOCKER_ORG="mailutest"
+    docker-compose -f tests/build.yml build
+fi
+
+
+# Note that in case of a PR, the branch is the one we are merging into
+if [ -n $DOCKER_UN ] && [ -n $DOCKER_PW ] && \
+{ [ "$TRAVIS_BRANCH" = "master" ] || [ "$TRAVIS_BRANCH" = "1.6" ]; }; then
+    docker login -u $DOCKER_UN -p $DOCKER_PW
+    docker-compose -f tests/build.yml push
+fi