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.

tests/tools/test_docker_find.py

@@ -46,3 +46,59 @@ class TestFindDocker:
         with patch("tools.environments.docker.shutil.which", return_value=None):
             second = docker_mod.find_docker()
         assert first == second == "/usr/local/bin/docker"
+
+    def test_env_var_override_takes_precedence(self, tmp_path):
+        """HERMES_DOCKER_BINARY overrides PATH and known-location discovery."""
+        fake_binary = tmp_path / "podman"
+        fake_binary.write_text("#!/bin/sh\n")
+        fake_binary.chmod(0o755)
+
+        with patch.dict(os.environ, {"HERMES_DOCKER_BINARY": str(fake_binary)}), \
+             patch("tools.environments.docker.shutil.which", return_value="/usr/bin/docker"):
+            result = docker_mod.find_docker()
+        assert result == str(fake_binary)
+
+    def test_env_var_override_ignored_if_not_executable(self, tmp_path):
+        """Non-executable HERMES_DOCKER_BINARY falls through to normal discovery."""
+        fake_binary = tmp_path / "podman"
+        fake_binary.write_text("#!/bin/sh\n")
+        fake_binary.chmod(0o644)  # not executable
+
+        with patch.dict(os.environ, {"HERMES_DOCKER_BINARY": str(fake_binary)}), \
+             patch("tools.environments.docker.shutil.which", return_value="/usr/bin/docker"):
+            result = docker_mod.find_docker()
+        assert result == "/usr/bin/docker"
+
+    def test_env_var_override_ignored_if_nonexistent(self):
+        """Non-existent HERMES_DOCKER_BINARY path falls through."""
+        with patch.dict(os.environ, {"HERMES_DOCKER_BINARY": "/nonexistent/podman"}), \
+             patch("tools.environments.docker.shutil.which", return_value="/usr/bin/docker"):
+            result = docker_mod.find_docker()
+        assert result == "/usr/bin/docker"
+
+    def test_podman_on_path_used_when_docker_missing(self):
+        """When docker is not on PATH, podman is tried next."""
+        def which_side_effect(name):
+            if name == "docker":
+                return None
+            if name == "podman":
+                return "/usr/bin/podman"
+            return None
+
+        with patch("tools.environments.docker.shutil.which", side_effect=which_side_effect), \
+             patch("tools.environments.docker._DOCKER_SEARCH_PATHS", []):
+            result = docker_mod.find_docker()
+        assert result == "/usr/bin/podman"
+
+    def test_docker_preferred_over_podman(self):
+        """When both docker and podman are on PATH, docker wins."""
+        def which_side_effect(name):
+            if name == "docker":
+                return "/usr/bin/docker"
+            if name == "podman":
+                return "/usr/bin/podman"
+            return None
+
+        with patch("tools.environments.docker.shutil.which", side_effect=which_side_effect):
+            result = docker_mod.find_docker()
+        assert result == "/usr/bin/docker"