Ignore the image entrypoint when creating job/sandbox containers
Given that user can now push any docker images (!204 (merged)), we will have troubles if the user sets an arbitrary entrypoint (eg: using the ENTRYPOINT instruction in the Dockerfile).
It is wiser to just ignore the entrypoint for the moment.
Note: in the future, we should remove Webapp.entrypoint and use the ENTRYPOINT defined in the image instead.