#507: check Docker host disk space before env create

[evgheni7]

Fixes #507

• Preflight check before creating the k3s-docker environment: runs df -Pk on Docker's data root (DockerRootDir) in a short-lived container using the k3s image, so it works for native Docker, Docker Desktop and remote DOCKER_HOST.
• Fails when less than 10% is free (k3s taints the node with disk-pressure below 5%; extra margin covers engine image pulls).
• If the check itself can't run, only a warning is logged.
• Image pull moved before network creation so the check runs before anything is created.