remove node-sass rebuild from Dockerfile.frontend (#11)

* native modules no longer used for sass so no rebuild required (adoption of `dart-sass`)
* https://github.com/wagtail/wagtail/pull/6033
* 7eeb44ad04

Dockerfile.frontend

@@ -6,5 +6,3 @@ COPY ./wagtail/package.json ./package.json
 RUN npm i npm@latest -g
 RUN npm --prefix / install
 RUN npm install npm-run-all gulp webpack /
-RUN node /node_modules/node-sass/scripts/install.js
-RUN npm rebuild node-sass