ci: use container-rpmbuilder image (#21)

CI images move off the packer-images monorepo to semver-released container-* repos.

- run RPM build steps on docker-internal/rpmbuilder:0.1.0-alma9

Reviewed-on: #21
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

.woodpecker/release.yaml

@@ -75,7 +75,7 @@ steps:
 
   # Package the built binary + generated shell completions into an RPM.
   - name: package
-    image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/rpmbuilder:0.1.0-alma9
     commands:
       - ./scripts/build-rpm.sh ${CI_COMMIT_TAG}
     depends_on: [build]