Re-use Dockerfile when building on Travis

.travis.yml

@@ -1,12 +1,10 @@
-language: go
+language: minimal
 
-go:
-  - stable
+services:
+  - docker
 
-# Go module behaviour is disabled by default for packages inside GOPATH.
-# Turn module support on explicitly:
-env:
-  - GO111MODULE=on
+script:
+  - make docker-build
 
 deploy:
   - provider: script