proposal: docker driver should detect and reject podman

In https://github.com/hashicorp/nomad-driver-podman/issues/165 we've encountered unfortunate cases where the `docker` driver detects Podman 4.0 as if it were actually Docker. But unfortunately the fidelity of Podman's API is not quite 100% and this has caused conflict between the `podman` and `docker` drivers if they're running on the same machine.

We should at the very least provide documentation describing how to disable the `docker` driver if you're using the `podman` driver. But it may also be worth having the `docker` driver try to fingerprint Podman and refuse to send it API calls. But this leads to a bit of a conundrum as Podman's Docker fidelity gets better over time; at some point we may want to allow the `docker` driver to "just work" with Podman instead.

Start by reviewing the docker driver entry point and the existing documentation for running the docker and podman drivers together. Determine whether the intended outcome is documentation, Podman fingerprinting, or both, and define how compatibility changes over time before implementation; the work is done when the chosen behavior prevents the reported driver conflict or clearly documents how to avoid it.

Written by the indexing model from the issue text.