I have attempted to do the same I have the LetsEncrypt docker container running with a reverse proxy setup to point back to the OMV webgui on the host machine.
It nearly works in the access log I can see all the java script being returned but in the source of the webpage returned I see a header of 404 though nothing renders on the page.
My setup
Duckdns -> 443 on my router -> 443 LetsEncrypt container -> 8080 on OMV host
So to call i use
https://mydomain.duckdns.org/omv
Anyone tried this?
here is my proxy conf
#omv base proxy
location /omv {
include /config/nginx/proxy.conf;
resolver 127.0.0.11 valid=30s;
proxy_pass http://192.168.0.23:8080;
}