Migrate build assets and scripts to deploy/docker structure

- Removed `install.sh` and moved Docker-related assets to `deploy/docker`.
- Updated all Dockerfile references from `build/` to `deploy/docker/`.
- Refactored paths in E2E tests, Makefile, and documentation for consistency with new directory structure.
- Added `HAPROXY_STATS_CORS_ORIGIN` environment variable in `docker-compose.yml`.

deploy/docker/assets/etc/easyhaproxy/haproxy/conf.d/README.md

@@ -0,0 +1,5 @@
+# Custom HAProxy Configuration
+
+Put `.cfg` files here to be included in the HAProxy configuration.
+
+These files will be available at `/etc/easyhaproxy/haproxy/conf.d/` inside the container.