ci/images: add image description

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+# Container images for CI
+
+## Image purpose
+
+### debian-11
+
+The main image used by shared runners to execute most CI builds and tests.
+
+### debian-bullseye
+
+Used to serve the same as `debian-11`. As of 2022-03-09, it is still used by
+some jobs (linters).
+
+### lxc-debian-11
+
+Very similar to the main image. The main difference is a custom base image
+which can be used for LXC runners and boots into systemd. It is useful to
+update it when `debian-11` gets updated, as it will allow some of the tests to
+be migrated to the LXC runners in the future (especially the
+unstable/problematic ones - pytests already migrated, deckard might be a good
+candidate).
+
+## Maintenance
+
+The `ci/images/` directory contains utility scripts to build, push or update
+the container images.
+
+```
+$ ./build.sh debian-11    # builds a debian-11 image locally
+$ ./push.sh debian-11     # pushes the local image into target registry
+$ ./update.sh debian-11   # utility wrapper that both builds and pushes the image
+$ ./update.sh */          # use shell expansion of dirnames to update all images
+```