Use qmk docker image for travis CI builds

util/travis_test.sh

@@ -19,4 +19,11 @@ if [ "$BRANCH" != "master" ] && [ "$NUM_IMPACTING_CHANGES" == "0" ]; then
     exit 0
 fi
 
+# if docker is installed - call make within the qmk docker image
+if command -v docker >/dev/null; then
+  function make() {
+    docker run --rm -e MAKEFLAGS="$MAKEFLAGS" -w /qmk_firmware/ -v "$PWD":/qmk_firmware qmkfm/qmk_firmware make "$@"
+  }
+fi
+
 make test:all