message: use explicit path to yarn, to work around base image issue
diff: @@ -37,7 +37,9 @@ WORKDIR /root/shadowbox
  COPY build/shadowbox/app app/
  COPY src/shadowbox/package.json .
  COPY src/shadowbox/yarn.lock .
  -RUN yarn install --prod
  +# TODO: Replace with plain old "yarn" once the base image is fixed:
  +# https://github.com/nodejs/docker-node/pull/639
  +RUN /opt/yarn-v$YARN_VERSION/bin/yarn install --prod
  # Create default state directory.
  RUN mkdir -p /root/shadowbox/persisted-state
  