Add option to use the original image entrypoint when creating session containers

[achimnol]

Sometimes users may want to run the original image entrypoint in addition to our entrypoint.sh.
It is "easy" to just execute yet another command from entrypoint.sh by passing the original entrypoint to the container via configuration files or environment variables, but there are several technical considerations, though:

• The image's original entrypoint command/script may assume the "root" privilege, while our entrypoint.sh switches to the plain work user.
• We should keep our kernel runner to become ready even if the original entrypoint script takes a long time or fails, to make the container "usable" at least.
  • The execution of the original entrypoint script should be done asynchronously and we should have a way to expose its execution log.