771: improve docs regarding usage in Docker

README.md

@@ -46,6 +46,10 @@ curl -fL --output /tmp/pie.phar https://github.com/php/pie/releases/latest/downl
   && sudo chmod +x /usr/local/bin/pie
 ```
 
+> [!TIP]
+> For usage in a Docker image build, we have a recommended usage example in
+> [usage#example-of-pie-working-in-a-dockerfile](./docs/usage.md#example-of-pie-working-in-a-dockerfile).
+
 ## Using PIE
 
 ### Installing a single extension using PIE

docs/usage.md

@@ -75,6 +75,13 @@ Docker image. Note that, like Composer, you need something like `unzip`, the
 [Zip](https://www.php.net/manual/en/book.zip.php) extension, or `git` to be
 installed.
 
+A quick summary of what the below example does:
+
+ - Mount PIE on `/usr/local/bin/pie` - this means PIE is only present at build time; it doesn't need to be in the runtime/final image
+ - Install `unzip` from apt; additional library dependencies for the extension could be added here
+ - Use `pie install` to install any relevant extension(s)
+ - Remove `unzip` and clear up `apt` lists
+
 ```Dockerfile
 FROM php:8.4-cli