Don't set MINIKUBE_IP if minikube isn't running

docker_compose_ensure_env: Conditionalize the setting of MINIKUBE_IP
to avoid a situation where the following ends up in the .env file:

TRAINDEV_WORKSPACE=
MINIKUBE_IP=* The control plane node must be running for this command
To start a cluster, run: "minikube start -p traindev"