feat: Support podman

[Junkern]

This will be an ongoing PR to be able to at least unofficially support podman, so that we can close #113

The goal is to be able to run the whole test suite locally (maybe in the future also have the test suit running here in Github)

Current steps:

• Use this PR as the baseline
• Init podman machine with the correct volume: podman machine init -v "$(pwd)"/scripts/testing:/mnt/testing
• spin up the testing infrastructure ./scripts/testacc_setup.sh
• Run some tests: DOCKER_HOST=unix:///Users/username/.local/share/containers/podman/machine/podman-machine-default/podman.sock TF_LOG=DEBUG TF_ACC=1 go test -v ./internal/provider -timeout 360s -run TestAccDockerContainer

Issues:

• Podman complains about our self-signed certificates for our local registries x509: certificate relies on legacy Common Name field, use SANs instead We created a registries.conf inside the podman VM to "whitelist" our insecure registry (see for exact commands https://gist.github.com/doytsujin/836701deb5e5521712628e575ea22c64)

Test Suite:

• TestAccDockerConfig: Apparently the service is not supported by podman: Podman does not support service: /v1.41/configs/create
• TestAccDockerContainer
  Many failing tests, often due to the same error as above
• TestAccDockerImage
  One failing test: TestAccDockerImage_name_attr_change due to force replacements of network_mode and pid_mode
• TestAccDockerNetwork
  Number of different errors
• TestAccDockerPlugin: It seems to me that docker plugins are not supported in podman
• TestAccDockerRegistryImage
  Some working tests, but many tests fail due to Bad Request for API route and version, e.g. TestAccDockerRegistryImageResource_whitelistDockerignore
• TestAccDockerSecret: All tests pass
• TestAccDockerService: Seems not to be supported by Podman: Podman does not support service: /v1.41/services/create
• TestAccDockerVolume: All tests pass
• TestAccDockerTag
  One failing test: TestAccDockerTag_no_local_image