docker: use default entrypoint and command where applicable (#2529)

A default entrypoint and command make it just a bit easier to use the images as
there is no longer a need for an explicit entrypoint. The exception is the
server image, which still requires the shell script to be specified.

server/Dockerfile

@@ -39,3 +39,5 @@ RUN npm link && npm cache clean --force
 VOLUME /usr/src/app/upload
 
 EXPOSE 3001
+
+ENTRYPOINT ["/bin/sh"]