Port docker.nix to the new Nix stuff

docker.nix

@@ -1,4 +1,4 @@
-# Builds a docker image with latest neuron from master branch.
+# Builds a docker image containing the neuron executable
 # Run as:
 #   docker load -i $(nix-build docker.nix)
 let
@@ -9,14 +9,11 @@ let
     imageDigest = "sha256:a969b4f238eab9298fe426fc90cc45efe01685431a419ca2907f10ddbbed2b7f";
     sha256 = "1zy4zwlka5c4hgjbgy8xh1ikyb3z2lwzfjwv8y0cyzx1danys4vk";
   };
-  neuron =
-    let src = builtins.fetchGit { url = "https://github.com/srid/neuron"; ref = "master"; };
-    in import src.outPath { gitRev = src.shortRev; };
 in
 
 pkgs.dockerTools.buildImage {
   fromImage = baseImage;
   name = "sridca/neuron";
   tag = "test";
-  contents = [ neuron ];
+  contents = [ (import ./.) ];
 }