nginx: Allow extending config with overrides

To facilitate this, the default redirect at / can be disabled, even if
the default remains at redirecting to the webmailer.

The extensions are within the host scope and are read from
$ROOT/overrides/nginx/*.conf.

core/nginx/Dockerfile

@@ -14,6 +14,7 @@ COPY *.py /
 
 EXPOSE 80/tcp 443/tcp 110/tcp 143/tcp 465/tcp 587/tcp 993/tcp 995/tcp 25/tcp 10025/tcp 10143/tcp
 VOLUME ["/certs"]
+VOLUME ["/overrides"]
 
 CMD /start.py