fix: auto-detect Podman to avoid host-gateway incompatibility (#117)

Podman doesn't support the `host-gateway` special value in extra_hosts,
which causes container startup failures on macOS with Podman Desktop.

Changes:
- Add docker-compose.docker.yml with extra_hosts override for Docker
- Update shannon script to detect Podman via `command -v podman`
- Skip extra_hosts override when Podman is detected

This ensures:
- Docker users (Linux): Get host.docker.internal working automatically
- Podman users (macOS): Base config works without modification

Co-authored-by: ajmallesh <ajmallesh@gmail.com>

docker-compose.docker.yml

@@ -0,0 +1,6 @@
+# Docker-specific overrides (not used with Podman)
+# This file is automatically included by the shannon script when running Docker
+services:
+  worker:
+    extra_hosts:
+      - "host.docker.internal:host-gateway"