ci: Update outdated comments, documentation and gitignore.

Use of `Dockerfile.template` and generated `tools/ci/images/` was
removed in 16067bc4fc.

.github/workflows/production-suite.yml

@@ -115,8 +115,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Base images are built using `tools/ci/Dockerfile.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:bionic
             name: Bionic production install with custom db name and user
             is_bionic: true

.github/workflows/zulip-ci.yml

@@ -16,26 +16,20 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # This docker image was created by a generated Dockerfile at:
-          #   tools/ci/images/bionic/Dockerfile
+          # Base images are built using `tools/ci/Dockerfile.prod.template`.
+          # The comments at the top explain how to build and upload these images.
           # Bionic ships with Python 3.6.
           - docker_image: zulip/ci:bionic
             name: Ubuntu 18.04 Bionic (Python 3.6, backend + frontend)
             os: bionic
             is_bionic: true
             include_frontend_tests: true
-
-          # This docker image was created by a generated Dockerfile at:
-          #   tools/ci/images/focal/Dockerfile
           # Focal ships with Python 3.8.2.
           - docker_image: zulip/ci:focal
             name: Ubuntu 20.04 Focal (Python 3.8, backend)
             os: focal
             is_focal: true
             include_frontend_tests: false
-
-          # This docker image was created by a generated Dockerfile at:
-          #   tools/ci/images/focal/Dockerfile
           # Bullseye ships with Python 3.9.2.
           - docker_image: zulip/ci:bullseye
             name: Debian 11 Bullseye (Python 3.9, backend)

.gitignore

@@ -34,9 +34,6 @@ package-lock.json
 
 /.dmypy.json
 
-# Dockerfiles generated for continuous integration
-/tools/ci/images
-
 # Generated i18n data
 /locale/en
 /locale/language_options.json

docs/testing/continuous-integration.md

@@ -92,11 +92,10 @@ are defined in the `aliases` section at the top of the file.
 
 ### Images
 
-GitHub Actions tests are run in containers started from images
-maintained by the Zulip team. These images are built by the `docker
-build` commands listed in `tools/ci/build-docker-images`. See the
-comments in `tools/ci/Dockerfile` and `tools/ci/Dockerfile.prod` for
-more information.
+GitHub Actions tests are run in containers that are spun off from the
+images maintained by Zulip team. The Docker images can be generated by
+running `tools/ci/build-docker-images`; see instructions at the top of
+`tools/ci/Dockerfile` for more information.
 
 ### Performance optimizations