Use distro release and Debian arch for the Docker apt repo

The repo line was hard-coded to bookworm/arm64 as a workaround before Docker published trixie packages. Derive it from ansible_distribution_release and map ansible_architecture to the Debian architecture name.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

vars/main.yml

@@ -1 +1,5 @@
 ---
+# ansible_architecture -> Debian package architecture
+docker_apt_arch:
+  aarch64: arm64
+  x86_64: amd64