Support SSH_LISTEN_PORT env var in docker app.ini template (#7829)

Signed-off-by: leigh capili <leigh@null.net>

docker/root/etc/s6/gitea/setup

     ROOT_URL=${ROOT_URL:-""} \
     DISABLE_SSH=${DISABLE_SSH:-"false"} \
     SSH_PORT=${SSH_PORT:-"22"} \
+    SSH_LISTEN_PORT=${SSH_LISTEN_PORT:-"${SSH_PORT}"} \
     LFS_START_SERVER=${LFS_START_SERVER:-"false"} \
     DB_TYPE=${DB_TYPE:-"sqlite3"} \
     DB_HOST=${DB_HOST:-"localhost:3306"} \

docker/root/etc/templates/app.ini

 ROOT_URL         = $ROOT_URL
 DISABLE_SSH      = $DISABLE_SSH
 SSH_PORT         = $SSH_PORT
+SSH_LISTEN_PORT  = $SSH_LISTEN_PORT
 LFS_START_SERVER = $LFS_START_SERVER
 LFS_CONTENT_PATH = /data/git/lfs
 

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

 * `RUN_MODE`: **dev**: For performance and other purposes, change this to `prod` when deployed to a production environment.
 * `SSH_DOMAIN`: **localhost**: Domain name of this server, used for the displayed clone URL in Gitea's UI.
 * `SSH_PORT`: **22**: SSH port displayed in clone URL.
+* `SSH_LISTEN_PORT`: **%(SSH\_PORT)s**: Port for the built-in SSH server.
 * `DISABLE_SSH`: **false**: Disable SSH feature when it's not available.
 * `HTTP_PORT`: **3000**: HTTP listen port.
 * `ROOT_URL`: **""**: Overwrite the automatically generated public URL. This is useful if the internal and the external URL don't match (e.g. in Docker).