ci: add docker-hub login step in CI (#601)

Author: SteveBunlon

.travis.yml

@@ -18,6 +18,8 @@ jobs:
       script: commitlint-travis
     - script: yarn lint
       name: Lint JavaScript
+    - script: echo "$DOCKER_PASSWORD" | docker login -u "$DOCKER_USERNAME" --password-stdin
+      name: Login on dockerhub
     - script: sleep 20; yarn test:coverage
       name: Tests
       # NOTICE: Handles code coverage reporting to Code Climate