Test out plugin

Author: nkammah

.buildkite/build_pr_pipeline.yml

@@ -6,6 +6,7 @@ steps:
       - elastic/vault-secrets#v0.0.2:
           path: "secret/ci/elastic-docs/docker.elastic.co" #"secret/ci/elastic-docs/docs_preview_cleaner"
           field: "username" #"value"
+          env_var: "DOCKER_REGISTRY_USERNAME"
 
       - elastic/vault-secrets#v0.0.2:
           path: "secret/ci/elastic-docs/docs_preview_cleaner"