Remove openssh-server from the images

[wilmardo]

Since running an ssh-server within an container is a considered a bad practice and should be avoided:

• https://medium.com/containers-101/docker-anti-patterns-ad2a1fcd5ce1
• https://jpetazzo.github.io/2014/06/23/docker-ssh-considered-evil/

Also when enabling the ssh-server now it runs as a second process within the container which goes against the core principle of Docker, one pid in the container. This can be resolved by using a supervisor (like tini) but I would like to raise the Why question :)

BTW. I am willing to refactor the current setup to resolve this issue, this issue is just to discuss if the PR will be accepted :)