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.
This also removes nixcrpkgs and OSX cross compilation in favour of
compiling on the target. x86_64/musl targets are still supported
on Linux.
All sources are now managed via niv (see nix/sources.json) and Haskell
package sets are provided/organised via IOHK's haskell.nix.
Some effort has been made to expose similar top-level attributes for
development, but in some cases there have been changes. Please see
the comments in the top-level default.nix and ci.nix files for usage.
