Add note about Private CA chain (#4901)

Update docs to include mention of the `DOCKER_DRIVER_PRIVATE_CA_PATH` env var

docs/install/docker/README.md

@@ -184,6 +184,16 @@ MIIEvQIBADANBgkqhkiG9w0BAQEFAASCBKcwggSjAgEAAoIBAQD
 "
 ```
 
+If you are using a private Certificate Authority then you will also need to tell the Node-RED instances to trust this CA.
+You can do this by includeing the `DOCKER_DRIVER_PRIVATE_CA_PATH` value in `.env` file. e.g. if the `ca.pem` file is located at `/usr/local/ssl/ca.pem`
+on the host machine
+
+```
+DOCKER_DRIVER_PRIVATE_CA_PATH="/usr/local/ssl/ca.pem"
+```
+
+
+
 ## Start FlowFuse platform
 
 **Note: Make sure all configuration are done above before proceeding.**
@@ -350,4 +360,4 @@ Full details on configuring the File Storage service are available [here](../fil
 
 - Bring the services down with `docker compose -p flowfuse down -v` (note the extra `-v` to delete all the volumes, only include this if you do not want to reuse this install)
 - Use `docker images` to list container images
-- Use `docker rmi [imagename]:[tag]` to remove all images that start with `flowfuse/`
+- Use `docker rmi [imagename]:[tag]` to remove all images that start with `flowfuse/`