I’t definietely not the best approach to do nested virtualization but in general it “should” work.
I saw similar issues in the past when connections between hosts in a docker setup (NC to Collabora - same as your issue - /hosting/discovery) failed/delayed as there was some issue using DNS resolution hosted on a container on the same host as well. The symptom was - DNS took long time ( up to 10-15 sec each request) the solution was to add public domain to docker “alias” of the reverse proxy see Probably DNS help with NC Docker + Collabora + Wireguard tunnel - #5 by wwe for details.
This might be little different in your case with separate reverse proxy but sounds similar.