Update Dockerfile

I believe this is the right folder now, after https://github.com/spotify/backstage/pull/895

Dockerfile

@@ -5,7 +5,7 @@ FROM nginx:mainline
 
 # The safest way to build this image is to use `yarn docker-build`
 
-COPY packages/app/build /usr/share/nginx/html
+COPY packages/app/dist /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
 CMD run.sh