I find part of the fun of dockerising small apps is in trying to get the image as small as possible with as few files in it as I can. This one looks like it still contains a lot of stuff that's not needed.
by just apt-get installig irssi, those 'few files' are even shared with other applications, making the whole thing even smaller and without the docker overhead.
I could go smaller using the light package, but I wanted a full featured Exim in the container. And I'm using the testing image so it keeps up with the latest version of Exim.
For example, my exim image https://hub.docker.com/r/grepular/exim4 is built like this: https://gitlab.com/grepular/docker-exim4/-/blob/main/Dockerf... - The final image only contains the necessary executables, shared lib files, CA certs, timezone files, a few other bits and nothing else.