tests: basic support for podman with testcontainers

[ahouene]

For all sorts of reasons one might prefer using Podman over Docker, but testcontainers won't pick that up out of the box.

This PR adds support for Podman in test.sh in order to fill the gap.

Podman will be used if DOCKER_HOST is undefined and no Docker endpoint is configured.
Setting SB_TESTS_FORCE_PODMAN to a non-empty value will use Podman regardless of Docker.

[Luit]

Personally, I'd prefer the script to look for a running non-root podman socket first, and fall back to Docker. Might not be as easy to set as default for Darwin, but would that be okay to default to on Linux?

[Luit]

Works on my side. The DOCKER_HOST override works as advertised as well, tested using eval $(minikube docker-env); ./test.sh.

I fixed this inside VSCode (actually VSCodium right now) with the following in my settings.json:

  "go.testEnvVars": {
    "TESTCONTAINERS_RYUK_DISABLED": "true",
    "DOCKER_HOST": "unix:///run/user/1000/podman/podman.sock"
  },

Ship it.