Dispatcher-image rebuild is manual

[Shadoom7]

Dispatcher-image needs to be manually rebuilt if any changes is made.

[jonathanmetzman]

I think we could solve this by adding code to run_experiment.py that builds dispatcher-image and pushes it to the docker registry.
An annoying thing about this is it will force rebuilding of base-image which can take a while and we may not want to use GCB for this because we will have to support local builds anyway for local experiments.
In the end I think we should implement GCB support for this though since I don't want the fuzzbench service to have to run on a fancy machine so that it can build the images in a timely manner.