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

The upstream woodpeckerci/plugin-docker-buildx image does not trust the
internal CA, so pushes/pulls against artifactapi over TLS fail. Swap both
the PR dry-run build step and the tag push step to the CA-baked custom
image published at docker-internal/plugin-docker-buildx:latest.

.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/agent-base
       dry_run: true

.woodpecker/docker.yaml

@@ -4,7 +4,7 @@ when:
 
 steps:
   - 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/agent-base