Hi,
The demos are behind a nginx reverse proxy (https://github.com/openemr/demo_farm_openemr/blob/master/docker/nginx/nginx.conf) but they use specialized dockers (Docker) that don’t have a context difference between the client and backend server.
Since alias won’t work, rec the other suggestion (change to DocumentRoot /var/www/localhost/htdocs). Would not change the actual directory structure since that would mess up the shared volumes and likely some other things.
Sounds like you might need to add the package. Inside the container, apk add --no-cache apache2-proxy should do it. Once that’s there you can properly use the directive, and I think it’ll work like you need it to even if that Apache isn’t the one running as a reverse proxy, just something talking to one.
Given that you’d want this change done in the container and always part of any images you use with Kubernetes, I think this is another argument for a custom image as described.