docker: explicitly set both user and group when running as unprivileged user.

This change amends eea99fe507.

https://docs.docker.com/engine/reference/builder/#user
  USER <user>[:<group>] or
  USER <UID>[:<GID>]

  The USER instruction sets the user name (or UID) and optionally the user group
  (or GID) to use when running the image and for any RUN, CMD and ENTRYPOINT
  instructions that follow it in the Dockerfile.

docker/Dockerfile

@@ -65,7 +65,7 @@ RUN \
     echo 'etherpad:x:65534:' > /etc/group && \
     chown -R etherpad:etherpad ./
 
-USER etherpad
+USER etherpad:etherpad
 
 EXPOSE 9001
 CMD ["node", "node_modules/ep_etherpad-lite/node/server.js"]