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>
