Added docker login to .travis.yml.

Author: ppouchin

.travis.yml

@@ -20,6 +20,7 @@ before_install:
   - git clone --recurse-submodules git://github.com/openmicroscopy/omero-test-infra .omero
   - export DOCKER_ARGS="-v $HOME/.m2:/root/.m2 -v $(pwd)/target:/src/target"
   - cp .travis.settings.xml $HOME/.m2/settings.xml
+  - echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
 
 after_success:
   - bash <(curl -s https://codecov.io/bash)