Add instructions for overriding docker nginx files

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2311>

docs/administrator_documentation/installation_docs/docker.md

@@ -140,7 +140,7 @@ That's it! Your Funkwhale pod is now up and running.
 
 ## 4. Set up your reverse proxy
 
-Funkwhale uses a reverse proxy to serve content to users. We use [Nginx](https://nginx.com) to serve this proxy. Follow this guide to install an Nginx configuration using details from your `.env` file.
+Funkwhale requires a reverse proxy to serve content to users. We recommend using [Nginx](https://nginx.com) to handle requests to your container. To do this:
 
 1. Install Nginx.
 
@@ -185,6 +185,52 @@ That's it! You've created your Nginx file. Run the following command to check th
 grep '${' /etc/nginx/sites-enabled/funkwhale.conf
 ```
 
+### Override default Nginx templates
+
+The frontend container ships default Nginx templates which serve content to the reverse proxy. These files read variables from your `.env` file to correctly serve content. In some cases, you might want to override these defaults. To do this:
+
+1. Create a `/srv/funkwhale/nginx` directory to house your files.
+
+   ```{code-block} sh
+   mkdir /srv/funkwhale/nginx
+   ```
+
+2. Download the Nginx template files to the `/srv/funkwhale/nginx` directory.
+
+   ```{code-block} sh
+   curl -L -o /srv/funkwhale/nginx/funkwhale.template "https://dev.funkwhale.audio/funkwhale/funkwhale/raw/${FUNKWHALE_VERSION}/deploy/docker.nginx.template"
+   curl -L -o /srv/funkwhale/nginx/funkwhale_proxy.conf "https://dev.funkwhale.audio/funkwhale/funkwhale/raw/${FUNKWHALE_VERSION}/deploy/docker.funkwhale_proxy.conf"
+   ```
+
+3. Make any changes you want to make to these files.
+4. Open your `docker-compose.yml` file in a text editor. For this example, we will use `nano`.
+
+   ```{code-block} sh
+   nano /srv/funkwhale/docker-compose.yml
+   ```
+
+5. Uncomment the lines in the `volumes` section of the `front` service by deleting the `#` in front of them.
+
+   ```{code-block} yaml
+   version: "3"
+   services:
+      front:
+         volumes:
+            # Uncomment if you want to use your previous nginx config, please let us
+            # know what special configuration you need, so we can support it with out
+            # upstream nginx configuration!
+            - "./nginx/funkwhale.template:/etc/nginx/conf.d/funkwhale.template:ro"
+            - "./nginx/funkwhale_proxy.conf:/etc/nginx/funkwhale_proxy.conf:ro"
+   ```
+
+6. Bring the `front` container up again to pick up the changes.
+
+   ```{code-block} sh
+   docker-compose up -d front
+   ```
+
+That's it! The container mounts your custom nginx files and uses its values to serve Funkwhale content. To revert to the default values, comment out the volumes by adding a `#` in front of them and bring the `front` container back up.
+
 ## 5. Set up TLS
 
 To enable your users to connect to your pod securely, you need to set up {abbr}`TLS (Transport Layer Security)`. To do this, we recommend using the <acme.sh> script.