Merge pull request #481 from spotify/rugvip/dockerfile

dockerfile: add comment with some instructions on how to run

Dockerfile

@@ -1,5 +1,10 @@
 FROM nginx:mainline
 
+# This dockerfile requires the app to be built on the host first, as it
+# simply copies in the build output into the image.
+
+# The safest way to build this image is to use `yarn docker-build`
+
 COPY packages/app/build /usr/share/nginx/html
 COPY docker/default.conf.template /etc/nginx/conf.d/default.conf.template
 COPY docker/run.sh /usr/local/bin/run.sh