docker: rootless image (#10154)

* docker: rootless image

* improve docs + remove check for write perm on custom

* add more info on ssh passtrough

* Add comment for internal ssh server in container config

docker/rootless/usr/local/bin/docker-entrypoint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+if [ -x /usr/local/bin/docker-setup.sh ]; then
+    /usr/local/bin/docker-setup.sh || { echo 'docker setup failed' ; exit 1; }
+fi
+
+if [ $# -gt 0 ]; then
+    exec "$@"
+else
+    exec /usr/local/bin/gitea -c ${GITEA_APP_INI} web
+fi