Add docker-compose config for host-based Fleet Server and new /ping route

- Introduce `docker-compose.fleet-host.yml` allowing Elastic Agent to connect to a host-installed Fleet Server.
- Add `pingRoute` to provide a simple `/ping` endpoint returning "pong" with a timestamp.

docker-compose.fleet-host.yml

@@ -0,0 +1,18 @@
+services:
+  # Do not start the in-container Fleet Server when using a host-installed Fleet Server
+  fleet-server:
+    profiles: [disabled]
+
+  # Point the Elastic Agent container to the host Fleet Server
+  elastic-agent:
+    environment:
+      - FLEET_URL=http://host.docker.internal:8220
+    # No change to 8200: APM intake stays exposed from this container
+
+## Usage (Windows + WSL host Fleet Server):
+## 1) Stop containerized Fleet services to free port 8220:
+##    docker compose stop fleet-server elastic-agent
+## 2) Install Fleet Server on WSL (see commands I provided), verify http://localhost:8220/api/status
+## 3) Start Elastic Agent pointing to host Fleet:
+##    docker compose -f docker-compose.yml -f docker-compose.fleet-host.yml up -d elastic-agent
+## 4) Verify in Kibana → Fleet → Agents: Fleet Server (host) healthy; Elastic Agent (container) healthy.