Allow using reprounzip without DIRECT access to Docker

[remram44]

By placing the run command in a script in the container instead of passing it to "docker run", it should be usable as ENTRYPOINT. Then you get a minimal Docker image that runs your experiment.

(note: need to go through a Dockerfile to set ENTRYPOINT)

Related to #117, #141

[remram44]

@seansummers mentioned at DASPOS that having access to the Docker client from reprounzip is unsafe and shouldn't be required. Simply outputting the Dockerfile with the correct ENTRYPOINT and letting the user both build the image and run the container should be an option.