ops: Fix the CI builder docker context

Docker context was set to the `Dockerfile` rather than the containing directory.

.circleci/config.yml

@@ -1383,6 +1383,6 @@ workflows:
           docker_file: ./ops/docker/ci-builder/Dockerfile
           docker_name: ci-builder
           docker_tags: <<pipeline.git.revision>>,latest
-          docker_context: ./ops/docker/ci-builder/Dockerfile
+          docker_context: ./ops/docker/ci-builder
           context:
             - oplabs-gcr
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