Add GraalPy 25 image replacing CPython

Multi-stage Dockerfile.graalpy on ghcr.io/graalvm/graalpy-community:25
that builds the granian and rloop PyO3 extensions from source against
GraalPy (no GraalPy wheels exist), installs the pure-Python deps, and
serves the unchanged RSGI app on the rloop event loop.

Adds docker-compose.graalpy.yml and a CI job publishing the image as
the *-graalpy tags on release/*.

.dockerignore

@@ -15,6 +15,8 @@ dist/
 build/
 tests/
 Dockerfile
+Dockerfile.graalpy
 .dockerignore
 opencode.json
 docker-compose.yml
+docker-compose.graalpy.yml