Installing additional OS packages when composing containers from docker hub images

Hi,

How do I install additional packages (such as traceroute) when creating containers from images pulled from docker hub? For example, if I pull jacobalberty/unifi and use the dockerfile and docker-compose file from https://github.com/jacobalberty/unifi-docker to create a container, what do I need to do to get traceroute installed on the base OS without having to do it after the container is up and running? I tried adding traceroute to the existing RUN line in the dockerfile but it seems to ignore that.

Thanks,

->g.