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/*.

docker-compose.graalpy.yml

@@ -0,0 +1,23 @@
+# GraalPy variant of the app. Standalone file (not an override) so it can be
+# run alongside the default compose file:
+#
+#   docker compose -f docker-compose.graalpy.yml up -d --build
+#
+# The app runs on GraalPy 25 instead of CPython; see Dockerfile.graalpy.
+services:
+  pyfconfig-graalpy:
+    build:
+      context: .
+      dockerfile: Dockerfile.graalpy
+    image: pyfconfig-graalpy:latest
+    restart: unless-stopped
+    environment:
+      # Public name shown in the HTML page and the curl examples; set to the
+      # deployment domain (e.g. "ifconfig.example.com").
+      SITE_NAME: ${SITE_NAME:-pyfconfig}
+      # Comma-separated CIDRs of trusted reverse proxies (e.g.
+      # "172.16.0.0/12"); required for correct client IPs when deployed
+      # behind a reverse proxy. Empty means no proxy is trusted.
+      TRUSTED_PROXY_CIDRS: ${TRUSTED_PROXY_CIDRS:-}
+    ports:
+      - "127.0.0.1:8080:8080"