The kamailio/kamailio docker image is using Debian. If you are familiar with Debian, then this image may be a choice to start with and extend.
The kamailio/kamailio-ci docker image is using Alpine. Alpine allows to create smaller and secure docker images.
Kamailio docker images sizes can be seen at:
Linux distributions details:
Note: you may need an account and be logged on on docker.com portals to access the links listed above.