Allow for easy use of Docker alternatives, like Podman

With the Docker Desktop license change, many are looking to alternatives like Podman.

It should be fairly easy to support changing out the name of the `docker` executable, and providing a `DOCKER` envVar.

With the `KIND_EXPERIMENTAL_PROVIDER` option, it should be even possible to use these with KinD.

No files or tests are named. Start by locating the `docker` executable usage and the handling of `KIND_EXPERIMENTAL_PROVIDER`; confirm how a `DOCKER` environment override should work with Podman and KinD. Done means the executable can be changed without breaking existing Docker usage and the documented alternative-provider path works.

Written by the indexing model from the issue text.