ci: use CA-baked plugin-docker-buildx image for artifactapi push

Buildkit in the upstream woodpeckerci/plugin-docker-buildx image does not trust the artifactapi internal Vault CA, so pushes to docker-internal fail on TLS.

- Switch the docker.yaml push step to the CA-baked custom plugin image
- Switch the build.yaml PR dry-run step to the same custom image

.woodpecker/build.yaml

@@ -3,7 +3,7 @@ when:
 
 steps:
   - name: docker-build
-    image: woodpeckerci/plugin-docker-buildx
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/plugin-docker-buildx:latest
     settings:
       repo: artifactapi.k8s.syd1.au.unkin.net/docker-internal/autobackup-operator
       dockerfile: Dockerfile.operator

.woodpecker/docker.yaml

@@ -6,7 +6,7 @@ steps:
   # No push credentials: the in-cluster runner has push access to the artifactapi
   # docker-internal registry (same credential-less pattern as jellyfin-ha).
   - name: docker
-    image: woodpeckerci/plugin-docker-buildx
+    image: artifactapi.k8s.syd1.au.unkin.net/docker-internal/plugin-docker-buildx:latest
     settings:
       registry: artifactapi.k8s.syd1.au.unkin.net
       repo: artifactapi.k8s.syd1.au.unkin.net/docker-internal/autobackup-operator