The jboss/keycloak does not contain jq even though the Dockerfile suggests that jq is supposed to be installed (and it is vital to be able to use the ./kcadm.sh script).
The reason is that the Dockerfile tries to install jq like this:
As stated in https://stackoverflow.com/a/45745410 it doesn't work to install jq and epel-release with one command since this results in:
I.e. epel-release must be present before yum install jq is run.
The fix is as simple as putting the yum install jq into a separate line.