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

@@ -3,19 +3,27 @@ language: node_js
 node_js:
   - "lts/*"
 
-install:
-  - "bin/installDeps.sh"
-  - "export GIT_HASH=$(git rev-parse --verify --short HEAD)"
-  - "npm install ep_test_line_attrib"
-
-script:
-  - "tests/frontend/travis/runner.sh"
+services:
+  - docker
 
 env:
   global:
     - secure: "WMGxFkOeTTlhWB+ChMucRtIqVmMbwzYdNHuHQjKCcj8HBEPdZLfCuK/kf4rG\nVLcLQiIsyllqzNhBGVHG1nyqWr0/LTm8JRqSCDDVIhpyzp9KpCJQQJG2Uwjk\n6/HIJJh/wbxsEdLNV2crYU/EiVO3A4Bq0YTHUlbhUqG3mSCr5Ec="
     - secure: "gejXUAHYscbR6Bodw35XexpToqWkv2ifeECsbeEmjaLkYzXmUUNWJGknKSu7\nEUsSfQV8w+hxApr1Z+jNqk9aX3K1I4btL3cwk2trnNI8XRAvu1c1Iv60eerI\nkE82Rsd5lwUaMEh+/HoL8ztFCZamVndoNgX7HWp5J/NRZZMmh4g="
 
+jobs:
+  include:
+    - name: "Test the Frontend"
+      install:
+        - "bin/installDeps.sh"
+        - "export GIT_HASH=$(git rev-parse --verify --short HEAD)"
+        - "npm install ep_test_line_attrib"
+      script:
+        - "tests/frontend/travis/runner.sh"
+    - name: "Test the Dockerfile"
+      script:
+        - "docker build ."
+
 notifications:
   irc:
     channels: