The docker-ish stacks don’t come with support for it. That’s also of how containers are built. Technically it’s possible, but the whole idea of such a container is that it’s code base is not changing and if it does a restart of the stack gets rid of it.
I understand how docker containers work, as you said technically it would be possible and I can think of 2 ways :
1.
So is there any way I can install the packages using CLI ?
That way I can take Zammad docker as base and build addons in the docker (custom docker image with addons built in).
2.
Second alternative could be mounting the Zammad to a NFS drive and mount them in all containers ?