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

@@ -1,18 +1,32 @@
 language: node_js
+
 node_js:
   - "lts/*"
-install:
-  - "bin/installDeps.sh"
-  - "export GIT_HASH=$(git rev-parse --verify --short HEAD)"
-  - "npm install ep_test_line_attrib"
-script:
-  - "tests/frontend/travis/runner.sh"
+
+jdk:
+  - oraclejdk6
+
+services:
+  - docker
+
 env:
   global:
     - secure: "WMGxFkOeTTlhWB+ChMucRtIqVmMbwzYdNHuHQjKCcj8HBEPdZLfCuK/kf4rG\nVLcLQiIsyllqzNhBGVHG1nyqWr0/LTm8JRqSCDDVIhpyzp9KpCJQQJG2Uwjk\n6/HIJJh/wbxsEdLNV2crYU/EiVO3A4Bq0YTHUlbhUqG3mSCr5Ec="
     - secure: "gejXUAHYscbR6Bodw35XexpToqWkv2ifeECsbeEmjaLkYzXmUUNWJGknKSu7\nEUsSfQV8w+hxApr1Z+jNqk9aX3K1I4btL3cwk2trnNI8XRAvu1c1Iv60eerI\nkE82Rsd5lwUaMEh+/HoL8ztFCZamVndoNgX7HWp5J/NRZZMmh4g="
-jdk:
-  - oraclejdk6
+
+jobs:
+  include:
+    - name: "Test frontend"
+      install:
+        - "bin/installDeps.sh"
+        - "export GIT_HASH=$(git rev-parse --verify --short HEAD)"
+        - "npm install ep_test_line_attrib"
+      script:
+        - "tests/frontend/travis/runner.sh"
+    - name: "Test Dockerfile"
+      script:
+        - "docker build ."
+
 notifications:
   irc:
     channels: