Docker also lets you run on any x86-64 system as well. I will be for sure updating and uploading a docker to the docker hub also, so it can be as easy as docker run calibrae-project/calibrd. The docker can be thus built on ubuntu 14.04, same as the appimage, and run on any linux, or within a docker.
There has recently been a privilege escalation exploit that can let the code escape from the container though - https://threatpost.com/docker-patches-container-escape-vulnerability/123161/ but since this project won't involve running arbitrary code, it should not be a problem. However, the appimage version will not have this problem run outside a container.
So it will be entirely arbitray, up to the operator how to deal with this.
Thanks - I didn't hear about that docker exploit, that's bad for bare-metal docker providers I guess, but won't affect most people who would use VPS or their own PCs.
I see, as long as there will remain a docker container, and preferably an easier to use one. It should not be anything like as difficult to set up a witness as it currently seems!