recovered ssh agent forwarding documentation

The documentation about ssh agent forwarding got lost when osxfs.md was deleted.
So I just want to add the old documentation again at some place where users might look for it.

This is the commit of the old documentation:
4dc6844905 (diff-124453101199ad6385df8981138b4ede623649e8b8b62ee7782be41830034b3a)

desktop/mac/networking.md

@@ -44,6 +44,32 @@ syntax for `-p` is `HOST_PORT:CLIENT_PORT`.
 
 See [Proxies](index.md#proxies).
 
+### SSH agent forwarding
+
+Docker Desktop for Mac allows you to use the host’s SSH agent inside a container. To do this:
+
+1. Bind mount the SSH agent socket by adding the following parameter to your `docker run` command:
+
+    `--mount type=bind,src=/run/host-services/ssh-auth.sock,target=/run/host-services/ssh-auth.sock`
+
+1. Add the `SSH_AUTH_SOCK` environment variable in your container:
+
+    `-e SSH_AUTH_SOCK="/run/host-services/ssh-auth.sock"`
+
+To enable the SSH agent in Docker Compose, add the following flags to your service:
+
+ ```yaml
+services:
+  web:
+    image: nginx:alpine
+    volumes:
+      - type: bind
+        source: /run/host-services/ssh-auth.sock
+        target: /run/host-services/ssh-auth.sock
+    environment:
+      - SSH_AUTH_SOCK=/run/host-services/ssh-auth.sock
+ ```
+
 ## Known limitations, use cases, and workarounds
 
 Following is a summary of current limitations on the Docker Desktop for {{Arch}}