feat: bake default white-label nginx.conf into the image

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.

docker-entrypoint.sh

@@ -1,19 +1,36 @@
 #!/usr/bin/env bash
 set -e
 
-echo "Generated nginx.conf:"
-echo ""
+# Defaults — overridable via env in compose / docker run.
+: "${NGINX_UPSTREAM_SHOP:=shop:9999}"
+: "${NGINX_UPSTREAM_CMS:=cms:5555}"
+: "${NGINX_CACHE_LIFETIME:=1h}"
+export NGINX_UPSTREAM_SHOP NGINX_UPSTREAM_CMS NGINX_CACHE_LIFETIME
+
+# Render the template only when the nginx.conf in place is the
+# upstream nginx-image stock default. Adapters that mount their own
+# config via compose `configs:` (e.g. mp) get the file replaced
+# before this script runs; we detect that by the absence of the
+# stock `worker_processes  auto;` line and leave that file alone.
+if [ -f /etc/nginx/nginx.conf.template ] \
+   && grep -q "worker_processes  auto;" /etc/nginx/nginx.conf 2>/dev/null; then
+  envsubst '${NGINX_UPSTREAM_SHOP} ${NGINX_UPSTREAM_CMS} ${NGINX_CACHE_LIFETIME}' \
+    < /etc/nginx/nginx.conf.template \
+    > /etc/nginx/nginx.conf
+  echo "→ libreshop default nginx.conf rendered from template"
+else
+  echo "→ adapter-provided nginx.conf detected; leaving untouched"
+fi
+
+mkdir -p /cache/shop /cache/uploads /cache/api
+
+echo "Effective nginx.conf:"
+echo "---"
 cat /etc/nginx/nginx.conf
+echo "---"
 
-mkdir -p /cache/shop
-mkdir -p /cache/uploads
-mkdir -p /cache/api
-
-echo "Starting nginx"
-echo "NGINX_DEBUG=${NGINX_DEBUG}"
-# Start Nginx
 if [ "$NGINX_DEBUG" = "true" ]; then
-  sed -i 's/error\.log warn/error.log debug/' /etc/nginx/nginx.conf
+  perl -i -pe 's|error_log /dev/stdout error|error_log /dev/stdout debug|' /etc/nginx/nginx.conf
   exec nginx-debug -g 'daemon off;'
 else
   exec nginx -g 'daemon off;'