ci/images/README: add KNOT_BRANCH explanation

diff --git a/ci/images/README.md b/ci/images/README.md
index d9efe0e8d1d24fdb2a1b212ea68954a21de5883d..3d09f6033241486cae2a07340cabdd079858905c 100644 (file)
--- a/ci/images/README.md
+++ b/ci/images/README.md
@@ -39,3 +39,11 @@ $ ./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
 ```
+
+By default, a branch of Knot DNS deemed to be stable is selected according to
+the `vars.sh` file. To build an image for a different Knot DNS branch, set the
+`KNOT_BRANCH` environment variable to the name of the branch, e.g.:
+
+```
+$ KNOT_BRANCH='3.2' ./update.sh debian-11
+```