kamailio is debian based. If you know debian then for you may be more easy extend this image.
This primary dist that is supported by kamailio team.

kamailio-ci is Alpine dist based. Alpine dist allow create more thin and secure docker images.
Please check kamailio images sizes

1) https://cloud.docker.com/u/kamailio/repository/docker/kamailio/kamailio/tags
2) https://cloud.docker.com/u/kamailio/repository/docker/kamailio/kamailio-ci/tags

and vulnerabilities

1) https://hub.docker.com/_/debian/scans/library/debian/9.6
2) https://hub.docker.com/_/alpine/scans/library/alpine/3.8