Fix building of Docker image with yarn

Yarn 0.19 throws strange ENOENT errors related to its cache, so using
0.18.

Yarn needs to be a dependency of this project, as otherwise it deletes
itself on yarn install and crashes.

package.json

@@ -73,6 +73,7 @@
     "svgo": "^0.7.1",
     "through2": "^2.0.1",
     "url-loader": "^0.5.7",
-    "webpack": "^2.2.1"
+    "webpack": "^2.2.1",
+    "yarn": "0.18.1"
   }
 }