Define the yaml anchor elsewhere, where it is still needed

.circleci/config.yml

@@ -72,8 +72,16 @@ workflows:
                 - "master"
 
     jobs:
-      - "build-image-debian-10":
+      # Every job that pushes a Docker image from Docker Hub needs to provide
-          <<: *DOCKERHUB_CONTEXT
+      # credentials.  Use this first job to define a yaml anchor that can be
+      # used to supply a CircleCI job context which makes Docker Hub
+      # credentials available in the environment.
+      #
+      # Contexts are managed in the CircleCI web interface:
+      #
+      #  https://app.circleci.com/settings/organization/github/tahoe-lafs/contexts
+      - "build-image-debian-10": &DOCKERHUB_CONTEXT
+          context: "dockerhub-auth"
       - "build-image-debian-9":
           <<: *DOCKERHUB_CONTEXT
       - "build-image-ubuntu-16-04":