Add docker build to travis

Require that the image build succeeds in order to pass ci

Author: mxmeinhold

.travis.yml

@@ -1,8 +1,11 @@
 language: python
 python:
   - "3.7"
+services:
+  - "docker"
 
 install:
   - "pip install -r requirements.txt"
 script:
   - "pylint --load-plugins pylint_quotes packet/routes packet"
+  - "docker build -t packet ."