Generate assets during build

Add an additional node image that is responsible for generating the CSS
and assets during build, which can then be copied into the build and
service stages.

Dockerfile

@@ -1,7 +1,25 @@
+ARG NODE_VERSION=14
+
+# A node container for generating the CSS and JS assets for the site, copying
+# in only the minimal files that are needed to do this. Once this is done, the
+# node dependencies can be installed using yarn, and Encore can be run to
+# generate the assets.
+FROM node:${NODE_VERSION} AS assets
+WORKDIR /app
+COPY package.json .
+COPY yarn.lock .
+COPY webpack.config.js .
+COPY source source
+RUN yarn install --frozen-lockfile
+
+FROM assets AS assets-build
+RUN yarn encore dev
+
 FROM opdavies/sculpin AS sculpin
 COPY composer.* ./
+WORKDIR /app/source
+COPY --from=assets-build /app/source/build build
 
 FROM opdavies/sculpin-serve AS sculpin-watch
 COPY --from=sculpin /app .
 COPY . .
-