feat: custom edge container Dockerfile with dispatcher dependencies

- Create docker/edge/Dockerfile with bash, jq, curl, git, docker-cli
- Create docker/edge/dispatcher.sh as placeholder no-op loop
- Update edge service to build from ./docker/edge instead of caddy:alpine image
- Mount Docker socket into edge container for dispatcher access
- Mount dispatcher.sh as read-only volume

docker/edge/dispatcher.sh

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+# dispatcher.sh — Edge task dispatcher placeholder
+#
+# TODO: Implement task polling and runner launching (#45)
+# Currently a no-op loop for future expansion.
+
+set -euo pipefail
+
+while true; do
+  # Placeholder: no-op loop, no logic yet
+  sleep 60
+done