feat(nginx): bake default white-label routing config, closes #1 (#2)

## Summary

- Add `nginx.conf.template` with full routing: admin/CMS surfaces → `cms:5555`, uploads → `cms:5555` (cached), `/api` → `shop:9999` (no cache), `/` → `shop:9999` (cached)
- Update `docker-entrypoint.sh` to render template via `envsubst` at startup; skip if adapter has already mounted its own config
- Update `Dockerfile` to `COPY nginx.conf.template` into image
- Bump CHANGELOG to v0.1.1

## Test plan

- [ ] Build image locally: `docker build -t libreshop/nginx:test libreshop/nginx/`
- [ ] Smoke test white-label default: no env overrides → confirm rendered `/etc/nginx/nginx.conf` contains `cms:5555` and `shop:9999`
- [ ] Smoke test env override: `NGINX_UPSTREAM_SHOP=shop:8080` → confirm substitution
- [ ] Smoke test adapter override: mount custom `nginx.conf` via compose `configs:` → confirm "adapter-provided nginx.conf detected" log line
- [ ] Deploy libreshop/demo with `nginx:v0.1.1` → confirm `/admin` returns 200

Closes #1

Co-authored-by: Michael Czechowski <michael.c@re-cinq.com>
Reviewed-on: #2

Dockerfile

@@ -1,8 +1,10 @@
-# Libre Shop - (Reverse) Proxy
-# Version 1.0.0
+# libreshop/nginx — reverse proxy fronting libreshop adapters.
 
 FROM nginx:1.26.2
 
+# White-label default config rendered at startup via envsubst.
+# Adapters override the whole nginx.conf via compose `configs:`.
+COPY nginx.conf.template /etc/nginx/nginx.conf.template
 COPY docker-entrypoint.sh /docker-entrypoint.sh
 RUN chmod +x /docker-entrypoint.sh