docs: document docker compose environment overrides

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

README.md

@@ -17,6 +17,14 @@
 
 Use the compiler to generate `content stream`, `resources`, and `bbox`, then map the output to your signature appearance DTO adapter.
 
+## Container environment overrides
+
+When using `docker-compose.yml`, these optional environment variables are supported:
+
+- `UID` and `GID` for container user/group mapping (default: `1000`).
+- `XDEBUG_ENABLED` and `XDEBUG_MODE` for Xdebug toggling (defaults: `0` and `off`).
+- `COMPOSER_CACHE_DIR` to override the Composer cache mount path (default: `/tmp/composer-cache`).
+
 ## License
 
 AGPL-3.0-or-later.