Document Docker healthcheck configuration

Added Docker healthcheck instructions for newt client.

manage/sites/install-site.mdx

@@ -199,6 +199,22 @@ services:
       - --endpoint https://app.pangolin.net
 ```
 
+#### Docker healthcheck
+The newt cliens suports healthchecks as described in [Configure Sites](/manage/sites/configure-site#param-health-file). This file can be used to do a
+healthcheck from within docker (compose file).
+
+We need to define the `HEALTH_FILE` environment variable as well as the healthcheck itself:
+```bash
+    environment:
+      - HEALTH_FILE=/tmp/healthy
+    healthcheck:
+      test: ["CMD-SHELL", "[ -f /tmp/healthy ]"]
+      interval: 30s
+      timeout: 5s
+      start_period: 30s
+      retries: 3
+```
+
 Start the service:
 
 ```bash
@@ -284,4 +300,4 @@ After installation, click on the router app link at the top of the page to confi
   <img src="/images/advantech_router_app_configure.png" width="400" centered/>
 </Frame>
 
-A complete config file is located at `/etc/newt/settings` on the router. You can edit this file directly to change credentials or add additional configuration options. After making changes, restart the router app to apply the new configuration. An example settings file can be found at: https://github.com/fosrl/newt/blob/main/packages/advantech/merge/etc/defaults
+A complete config file is located at `/etc/newt/settings` on the router. You can edit this file directly to change credentials or add additional configuration options. After making changes, restart the router app to apply the new configuration. An example settings file can be found at: https://github.com/fosrl/newt/blob/main/packages/advantech/merge/etc/defaults