* add podman support to docker_build.sh script

* break out runtime into the RUNTIME variable
* allows RUNTIME to be set by the user
* decides on docker or podman if docker isn't avaible
* rewrote check for docker-machine to account only for docker runtime
* put --user arg into a variable only to be used with docker
  this is not needed with podman as podman maps the containers root id
  to the users id.

* add podman to getting_started_docker documentation