ci: Update outdated comments to generated Dockerfiles.

These were missed in the doc updates in d78723b6e8, which were for
behavior changes in 16067bc4fc.

.github/workflows/production-suite.yml

@@ -33,8 +33,8 @@ jobs:
     name: Bionic production build
     runs-on: ubuntu-latest
 
-    # This docker image was created by a generated Dockerfile at:
-    #   tools/ci/images/bionic/Dockerfile
+    # Docker images are built from 'tools/ci/Dockerfile'; the comments at
+    # the top explain how to build and upload these images.
     # Bionic ships with Python 3.6.
     container: zulip/ci:bionic
     steps:
@@ -220,8 +220,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Base images are built using `tools/ci/Dockerfile.prod.template`.
-          # The comments at the top explain how to build and upload these images.
+          # Docker images are built from 'tools/ci/Dockerfile'; the comments at
+          # the top explain how to build and upload these images.
           - docker_image: zulip/ci:buster-3.4
             name: 3.4 Version Upgrade
             is_focal: true

tools/ci/Dockerfile

@@ -7,6 +7,8 @@
 # To rebuild from this file for a given release, say Ubuntu 18.04 bionic:
 #   docker build . --build-arg=BASE_IMAGE=ubuntu:18.04 --pull --tag=zulip/ci:bionic
 #   docker push zulip/ci:bionic
+#
+# tools/ci/build-docker-images will rebuild all images, but not push them.
 
 ARG BASE_IMAGE
 FROM $BASE_IMAGE