I’m not sure if forwarding a different protocol works through the Nginx proxy, which supports HTTP to my knowledge. SSH is a different protocol and may need different proxy types (i.e. a SOCKS proxy).
Please share the configuration from the Nginx proxy and how it is deployed with docker compose.
So GitLab does not know that the default SSH port 22 was changed to 2222 on the GitLab server.
We had a similar discussion recently in Some questions about the GitLab - #15 by dnsmichi and the setting gitlab_shell_ssh_port - changing its value to 2222 should solve the git clone URL and use the correct port for cloning via SSH.