Able to set SSH port when install, update Docker docs

conf/locale/locale_en-US.ini

@@ -76,6 +76,8 @@ run_user = Run User
 run_user_helper = The user must have access to Repository Root Path and run Gogs.
 domain = Domain
 domain_helper = This affects SSH clone URLs.
+ssh_port = SSH Port
+ssh_port_helper = Port number which your SSH server is using, leave it empty to disable SSH feature.
 http_port = HTTP Port
 http_port_helper = Port number which application will listen on.
 app_url = Application URL