feat: option --no-push

[schlichtanders]

this is useful for minikube, where you can directly build using the minikube docker instead of setting up and pushing to a dummy registry.

See here for the minikube instructions about how to directly build inside minikube.

I tested them locally and could speed up my build process significantly:

• With push, after stopping and restarting minikube, the images (several GB) need to be send to the minikube cluster again. This is a common case, kind of every day, as I like to stop the minikube if I don't need it.
• Without push (i.e. with the new --no-push enabled), no pushing is needed and the cache of the minikube docker can be successfully used. julia_pod is super fast even after restarting minikube.
• Mind that when deleting minikube cluster and recreating it, the cache of the minikube docker is lost and the rebuilding takes very long. But this is very rare, only needed if you are still experimenting about which minikube config like cpus and memory are good for your local dev environment.

[schlichtanders]

Thank you for thumbs up.

I added documentation to the README.md (mind that my autosave config removed all trailing whitespace, the actual changes are rather a few. It helps to set the github's option to ignore whitespace changes - click on the gear symbol at the top)

I hope it is ready for merge now :slightly_smiling_face: