woodpecker: add plugin-docker-buildx CI ServiceAccount (#368)

Why: the new unkin/plugin-docker-buildx repo's pipelines run their buildah build/push step under `serviceAccountName: plugin-docker-buildx`, which must exist in the woodpecker namespace.

How: add serviceaccount_plugin_docker_buildx.yaml + wire it into kustomization.yaml, mirroring the existing per-repo woodpecker SAs.

Pairs with unkin/plugin-docker-buildx#1.

Reviewed-on: #368
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

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