Added docker login

.travis.yml

@@ -18,6 +18,12 @@ before_install:
   - mkdir -p $HOME/.docker
   - echo '{"experimental":"enabled"}' | sudo tee $HOME/.docker/config.json
   - sudo service docker start
+  - >
+    if [ ! -z "${DOCKERHUB_PULL_USERNAME:-}" ]; then
+      set +x
+    echo "${DOCKERHUB_PULL_PASSWORD}" | docker login --username "${DOCKERHUB_PULL_USERNAME}" --password-stdin
+      set -x
+    fi
 
 install:
 - docker --version