It's a bit odd to use a custom docker image, rather than the one maintained by nginx Inc though:

https://github.com/signalapp/Signal-TLS-Proxy/blob/main/ngin...

Vs

https://github.com/nginxinc/docker-nginx

For one, this is 5 versions behind (1.18 vs 1.23).

In general seems caddy or haproxy might be a better fit - but nginx is a perfectly fine choice I suppose.