docker: avoid rebuilding CSS/JS assets on every commit.

Restructure the Dockerfile and the CSS/JS files so that we only rebuild
the CSS and JS when they change, not on every commit.

Before it took several minutes to rebuild the Docker image after every
commit, even when the JS/CSS files didn't change. This also made pulling
images slower.

This requires refactoring the CSS and JS to not use embedded Ruby (ERB)
templates, since this made the CSS and JS dependent on the Ruby
codebase, which is why we had to rebuild the assets after every Ruby
change.

config/docker/Dockerfile.danbooru

@@ -35,32 +35,37 @@ RUN \
   rm -rf "$(gem environment gemdir)/cache" && \
   find "$(gem environment gemdir)" -name "*.o" -delete
 
-COPY .yarn package.json yarn.lock ./
+COPY .yarnrc.yml package.json yarn.lock ./
+COPY .yarn/ ./.yarn/
 RUN yarn install
 
-COPY . .
-
-ARG SOURCE_COMMIT
-RUN echo "$SOURCE_COMMIT" > REVISION
-
-
-
-FROM development AS assets
+COPY postcss.config.js Rakefile ./
+COPY bin/rails bin/webpack ./bin/
+COPY config/application.rb config/boot.rb config/danbooru_default_config.rb config/webpacker.yml ./config/
+COPY config/webpack/ ./config/webpack/
+COPY public/images ./public/images
+COPY public/fonts ./public/fonts
+COPY app/components/ ./app/components/
+COPY app/javascript/ ./app/javascript/
 
 RUN \
   bin/rails assets:precompile && \
   rm -rf \
     node_modules .yarn/cache tmp /usr/local/share/.cache/yarn \
     /root/.yarn/berry/cache /root/.bundle/cache /root/.npm && \
-  asdf uninstall nodejs && \
-  ln -s /tmp tmp
+  ln -s /tmp tmp && \
+  ln -s packs public/packs-test
 
 
 
 FROM base as production
 
 COPY --from=development /root /root
-COPY --from=assets /danbooru /danbooru
+COPY --from=development /danbooru /danbooru
+
+COPY . .
+ARG SOURCE_COMMIT
+RUN echo "$SOURCE_COMMIT" > REVISION
 
 USER danbooru
 ENTRYPOINT ["tini", "--"]