Vere will not run a ship if STDIN is closed, unless passed the `-t`
flag. Hosted docker containers will not be interactive, but we would
like the docker image to be usable locally and interactively as well.
Thus, we check for STDIN being closed in the shell prior to launching
Vere, and if it is closed, pass the `-t` flag.
This commit adds the `docker-image` attribute to the main Nix
entrypoint, invoking `nix/pkgs/docker-image` which will build
a 'smart' docker image that can load keyfiles or a pier and
boot a ship
It includes a README for the official docker image, suitable
for posting as the README to a Docker Hub or similar docker
image repository.
