The image used to ship just nginx + an entrypoint, with no routing
config — adapters had to mount their own nginx.conf via compose
`configs:` for anything to work. This broke the standalone
libreshop/demo preview (`curl /admin` returned 404).
Add a default nginx.conf.template that routes the standard libreshop
surface (`/admin`, `/uploads`, `/api`, `/`, ...) to
`${NGINX_UPSTREAM_CMS:-cms:5555}` / `${NGINX_UPSTREAM_SHOP:-shop:9999}`,
plus health and forwarded-headers handling. The entrypoint runs
envsubst at startup, then nginx.
Adapter override path is preserved: if compose `configs:` already
replaced /etc/nginx/nginx.conf with a non-stock file (detected via
the absence of the upstream nginx `worker_processes auto;` line),
the entrypoint leaves it untouched.
Closes#1.
