feat: entry-level Podman support — find_docker() + rootless entrypoint (#10066)

- find_docker() now checks HERMES_DOCKER_BINARY env var first, then
  docker on PATH, then podman on PATH, then macOS known locations
- Entrypoint respects HERMES_HOME env var (was hardcoded to /opt/data)
- Entrypoint uses groupmod -o to tolerate non-unique GIDs (fixes macOS
  GID 20 conflict with Debian's dialout group)
- Entrypoint makes chown best-effort so rootless Podman continues
  instead of failing with 'Operation not permitted'
- 5 new tests covering env var override, podman fallback, precedence

Based on work by alanjds (PR #3996) and malaiwah (PR #8115).
Closes #4084.

.env.example

@@ -145,6 +145,10 @@
 # Only override here if you need to force a backend without touching config.yaml:
 # TERMINAL_ENV=local
 
+# Override the container runtime binary (e.g. to use Podman instead of Docker).
+# Useful on systems where Docker's storage driver is broken or unavailable.
+# HERMES_DOCKER_BINARY=/usr/local/bin/podman
+
 # Container images (for singularity/docker/modal backends)
 # TERMINAL_DOCKER_IMAGE=nikolaik/python-nodejs:python3.11-nodejs20
 # TERMINAL_SINGULARITY_IMAGE=docker://nikolaik/python-nodejs:python3.11-nodejs20