You’re building the container with launcher with the stuff from Serve Discourse from a subfolder (path prefix) instead of a subdomain before you get docker-compose to launch it?
What’s the proxy? Maybe just launch discourse with launcher and use docker-args to set other stuff that is needed? For traefik, I do something like this:
--docker-args "-l traefik.frontend.rule=Host:{{discourse_hostname}};PathPrefix:/{{discourse_subfolder}} \
-l traefik.frontend.entryPoints=https \
-l traefik.backend={{discourse_shortname}} \
-l traefik.port=80 \
{{ docker_extra_args | default('')}}"