Add docs on passing SSH through to container (#5019)

* Add docs on passing SSH through to container

Wasn't clear how to pass SSH connections through the container, found a
few blog posts which described roughly the same procedure I've
documented here.

Credit should really go to:

* https://blog.sakuragawa.moe/gitea-in-docker-container-and-sharing-ssh-with-host/
* http://www.ateijelo.com/blog/2016/07/09/share-port-22-between-docker-gogs-ssh-and-local-system

Signed-off-by: Dane Elwell <dane.elwell@ukfast.co.uk>

* Add note to resolve @lafriks feedback

docs/content/doc/installation/with-docker.en-us.md

@@ -280,3 +280,67 @@ docker-compose pull
 # Start a new container, automatically removes old one
 docker-compose up -d
 ```
+
+# SSH Container Passthrough
+
+Since SSH is running inside the container, you'll have to pass SSH from the host to the
+container if you wish to use SSH support. If you wish to do this without running the container
+SSH on a non-standard port (or move your host port to a non-standard port) you can forward
+SSH connections destined for the container with a little extra setup.
+
+This guide assumes that you have created a user on the host called `git` which shares the same 
+UID/GID as the container values `USER_UID`/`USER_GID`. You should also create the directory
+`/var/lib/gitea` on the host, owned by the `git` user and mounted in the container, e.g.
+
+```
+  services:
+    server:
+      image: gitea/gitea:latest
+      environment:
+        - USER_UID=1000
+        - USER_GID=1000
+      restart: always
+      networks:
+        - gitea
+      volumes:
+        - /var/lib/gitea:/data
+      ports:
+        - "3000:3000"
+        - "127.0.0.1:2222:22"
+```
+
+You can see that we're also exposing the container SSH port to port 2222 on the host, and binding this
+to 127.0.0.1 to prevent it being accessible external to the host machine itself.
+
+On the **host**, you should create the file `/app/gitea/gitea` with the following contents and
+make it executable (`chmod +x /app/gitea/gitea`):
+
+```
+#!/bin/sh
+ssh -p 2222 -o StrictHostKeyChecking=no git@127.0.0.1 "SSH_ORIGINAL_COMMAND=\"$SSH_ORIGINAL_COMMAND\" $0 $@"
+```
+
+Your `git` user needs to have an SSH key generated:
+
+```
+sudo -u git ssh-keygen -t rsa -b 4096 -C "Gitea Host Key"
+```
+
+Still on the host, symlink the container `.ssh/authorized_keys` file to your git user `.ssh/authorized_keys`.
+This can be done on the host as the `/var/lib/gitea` directory is mounted inside the container under `/data`:
+
+```
+ln -s /var/lib/gitea/git/.ssh/authorized_keys /home/git/.ssh/authorized_keys
+```
+
+Then echo the `git` user SSH key into the authorized_keys file so the host can talk to the container over SSH:
+
+```
+echo "no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty $(cat /home/git/.ssh/id_rsa.pub)" >> /var/lib/gitea/git/.ssh/authorized_keys
+```
+
+Now you should be able to use Git over SSH to your container without disrupting SSH access to the host.
+
+Please note: SSH container passthrough will work only if using opensshd in container, and will not work if
+`AuthorizedKeysCommand` is used in combination with setting `SSH_CREATE_AUTHORIZED_KEYS_FILE=false` to disable
+authorized files key generation.