chore(deps): update docker.elastic.co/ci-agent-images/serverless-docker-builder docker tag to v0.2.1 (#161)

Co-authored-by: elastic-renovate-prod[bot] <174716857+elastic-renovate-prod[bot]@users.noreply.github.com>

Author: elastic-renovate-prod[bot]

.buildkite/pipeline.yml

@@ -55,7 +55,7 @@ steps:
       - label: ":docker: :seedling: Trigger Image Creation"
         command: "make -C /agent generate-docker-images"
         agents:
-          image: "docker.elastic.co/ci-agent-images/serverless-docker-builder:0.2.0"
+          image: "docker.elastic.co/ci-agent-images/serverless-docker-builder:0.2.1"
 
   - wait