ci: Test upgrades from the latest minor release.

.github/workflows/production-suite.yml

@@ -235,6 +235,10 @@ jobs:
             name: 3.4 Version Upgrade
             os: buster
 
+          - docker_image: zulip/ci:bullseye-4.7
+            name: 4.7 Version Upgrade
+            os: bullseye
+
     name: ${{ matrix.name  }}
     container:
       image: ${{ matrix.docker_image }}

docs/subsystems/release-checklist.md

@@ -76,6 +76,11 @@ preparing a new release.
 
 ### Post-release
 
+- Update the CI targets in `.github/workflows/production-suite.yml` to
+  include upgrades from the most recent point releases from the last
+  two series -- e.g after releasing 4.8, both `main` and `4.x` should
+  should test upgrades from 3.4 and 4.8, and after releasing 5.0 both
+  `main` and `5.x` should test upgrades from 4.8 and 5.0.
 - Following a major release (e.g. 4.0):
   - Create a release branch (e.g. `4.x`).
   - On the release branch, update `ZULIP_VERSION` in `version.py` to

tools/ci/build-docker-images

@@ -6,3 +6,4 @@ docker build . --build-arg=BASE_IMAGE=ubuntu:20.04 --pull --tag=zulip/ci:focal
 docker build . --build-arg=BASE_IMAGE=debian:10 --pull --tag=zulip/ci:buster
 docker build . --build-arg=BASE_IMAGE=debian:bullseye --pull --tag=zulip/ci:bullseye
 docker build . -f Dockerfile.prod --build-arg=BASE_IMAGE=zulip/ci:buster --build-arg=VERSION=3.4 --tag=zulip/ci:buster-3.4
+docker build . -f Dockerfile.prod --build-arg=BASE_IMAGE=zulip/ci:bullseye --build-arg=VERSION=4.7 --tag=zulip/ci:bullseye-4.7