woodpecker: add plugin-docker-buildx CI ServiceAccount

The unkin/plugin-docker-buildx repo's release pipeline runs a buildah step under
serviceAccountName plugin-docker-buildx to build+push the CA-baked buildx plugin
image to artifactapi's docker-internal registry.

apps/base/woodpecker/kustomization.yaml

@@ -10,6 +10,7 @@ resources:
   - serviceaccount_autobackup_operator_ci.yaml
   - serviceaccount_ghp.yaml
   - serviceaccount_kea_operator_ci.yaml
+  - serviceaccount_plugin_docker_buildx.yaml
   - serviceaccount_jellyfin_ha_src.yaml
   - serviceaccount_terraform_artifactapi.yaml
   - serviceaccount_terraform_authentik.yaml

apps/base/woodpecker/serviceaccount_plugin_docker_buildx.yaml

@@ -0,0 +1,6 @@
+---
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: plugin-docker-buildx
+  namespace: woodpecker