Docs for installing in Docker builds

Author: Wirone

docs/usage.md

@@ -27,6 +27,19 @@ system:
 sudo curl -L --output /usr/local/bin/pie https://github.com/php/pie/releases/latest/download/pie.phar && sudo chmod +x /usr/local/bin/pie
 ```
 
+### Docker installation
+
+PIE is published as binary-only Docker image, so you can install it easily during your Docker build:
+
+```Dockerfile
+COPY --from=ghcr.io/php/pie:latest-bin /pie /usr/bin/pie
+```
+
+Instead of `latest` you can also use explicit versions like `x.y.z-bin`, `x.y-bin` or `x-bin`, depending on stability level you want to achieve.
+
+> [!IMPORTANT]  
+> Binary-only images don't include PHP runtime so you can't use them for _running_ PIE. This is just an alternative way of distributing PHAR file, you still need to satisfy PIE's runtime requirements on your own.
+
 ## Prerequisites for PIE
 
 Running PIE requires PHP 8.1 or newer. However, you may still use PIE to install