Complete action runtime rewrite

tests/e2e/test_dotfiles_e2e.py

@@ -22,7 +22,14 @@ IMAGE_TAG = "flow-e2e:test"
 
 def _pick_runtime() -> str | None:
     for binary in ("podman", "docker"):
-        if shutil.which(binary):
+        if not shutil.which(binary):
+            continue
+        result = subprocess.run(
+            [binary, "version"],
+            capture_output=True,
+            text=True,
+        )
+        if result.returncode == 0:
             return binary
     return None