mksh: docker: not found

I see no reason to install docker to run simh when you can just install simh to run simh.

docker is mostly used to package and install simh.

dunno man, the following seems easier and cleaner to me:

   pacman -S simh
   
   apt install simh

   pacman -Rns simh

?? On that note is there anything that will get rid of unused docker layers when you get rid of an image? sort of like -Rns in pacman? or aptitude remove except for docker images?

AFAIK layers are automatically garbage collected if you delete all images and containers that depend on them. Bear in mind that containers are not automatically deleted after shutting down, unless you give the --rm argument.

You can list them with docker ps -a, and delete all containers by running docker rm $(docker ps -aq).

Newer versions have a prune option. Older ones there are shell alias recipes you can find on the web