Add CA-baked plugin-docker-buildx image + buildah CI

Bake the internal Vault-PKI CA into a rebuild of
woodpeckerci/plugin-docker-buildx so k8s buildx jobs can push to
artifactapi's docker-internal registry over TLS.

- Dockerfile: copy unkin-ca.crt into /etc/docker/certs.d/<registry>/ca.crt
  and the system trust store
- unkin-ca.crt: vault-ca-cert bundle (root + intermediate)
- .woodpecker/build.yaml: PR gate, buildah build-only
- .woodpecker/release.yaml: on v* tag, buildah build + credless push to
  docker-internal with --tls-verify=false (bootstrap)

Dockerfile

@@ -0,0 +1,6 @@
+FROM woodpeckerci/plugin-docker-buildx:latest
+# buildkit reads this to trust artifactapi's internal-CA registry on push
+COPY unkin-ca.crt /etc/docker/certs.d/artifactapi.k8s.syd1.au.unkin.net/ca.crt
+# fallback: system trust store (docker login / other code paths)
+COPY unkin-ca.crt /usr/local/share/ca-certificates/unkin-ca.crt
+RUN cat /usr/local/share/ca-certificates/unkin-ca.crt >> /etc/ssl/certs/ca-certificates.crt || true