Based on: https://www.bridgetownrb.com/docs/deployment#dynamic-site

Ideas for future improvements in the Dockerfile:
  * image size could be reduced by moving build-essential to the "gem" step
  * COPY --from=base is unnecessary when preceeded by FROM base
  * Moving yarn install to before COPY . . and after a new COPY package.json .
    would make yarn install only be run when package.json changes.
  * Moving yarn install to a separate build step would enable yarn install
    and gem install to be run concurrently