e2e: create a local private registry for docker and podman tests

[24215](https://github.com/hashicorp/nomad/pull/24215) tried introducing new docker e2e tests to make sure we don't cause any auth regressions, but sadly this approach was flawed—we can't change Nomad client configs _before_ creating the registry or auth configs. This has to be done via terraform during the provisioning of the cluster (same for podman), not in the tests themselves.

Start by reading the approach described in PR 24215 and inspect the repository's Terraform used to provision the e2e cluster. Determine where Docker and Podman clients and authentication configuration are created during provisioning. Done means the provisioning creates a local private registry and auth setup before the e2e tests run for both runtimes.

Written by the indexing model from the issue text.