ci: test the Dockerfile

Add a `Test Dockerfile` job to Travis that checks the `docker build` exit
code.
More useful tests can be added later.

.travis.yml

@@ -3,6 +3,9 @@ language: node_js
 node_js:
   - "lts/*"
 
+services:
+  - docker
+
 env:
   global:
     - secure: "WMGxFkOeTTlhWB+ChMucRtIqVmMbwzYdNHuHQjKCcj8HBEPdZLfCuK/kf4rG\nVLcLQiIsyllqzNhBGVHG1nyqWr0/LTm8JRqSCDDVIhpyzp9KpCJQQJG2Uwjk\n6/HIJJh/wbxsEdLNV2crYU/EiVO3A4Bq0YTHUlbhUqG3mSCr5Ec="
@@ -17,6 +20,9 @@ jobs:
         - "npm install ep_test_line_attrib"
       script:
         - "tests/frontend/travis/runner.sh"
+    - name: "Test the Dockerfile"
+      script:
+        - "docker build ."
 
 notifications:
   irc: