Follow the builder image to the build org

A package namespace does not travel with the repo and gets no redirect,
so the image had stayed in internal/ after the org reorganization. It
now lives under build/, next to this repo.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -49,8 +49,8 @@ then triggers
 
 ```sh
 docker build --platform linux/amd64 -f builder.Dockerfile \
-  -t git.teletypegames.org/internal/phaser-builder:latest .
-docker push git.teletypegames.org/internal/phaser-builder:latest
+  -t git.teletypegames.org/build/phaser-builder:latest .
+docker push git.teletypegames.org/build/phaser-builder:latest
 ```
 
 Detailed documentation lives on the wiki: `/development/phaser`.