Do a Docker login on Travis

.travis.yml

@@ -25,6 +25,7 @@ before_deploy:
       sudo -E apt-get -yq update;
       sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install docker-ce=${DOCKER_VERSION}*;
       docker version;
+      echo "${DOCKERHUB_PASSWORD}" | docker login -u "${DOCKERHUB_USERNAME}" --password-stdin;
       make build-image;
       if [ "$TRAVIS_TAG" ]; then
         make release-packages;
@@ -54,4 +55,4 @@ deploy:
     skip_cleanup: true
     on:
       repo: traefik/traefik
-      all_branches: true
+      all_branches: true