nix: Add nix-shell for convenient dev

This is meant for use with `nix-shell` in the project root or
`nix-shell /path/to/project/root`

Author: jsoo1

nix/overlay.nix

@@ -4,5 +4,11 @@ final: prev: {
     # nix runtime but not end up in a derivation. passthru.tests is
     # one very common attribute.
     passthru.tests.basic-raft = final.callPackage ./nixos-tests/basic-raft { };
+
+    # My personal preference is to put a dev shell in there, too
+    passthru.shell = final.mkShell {
+      name = "null-db-shell";
+      inputsFrom = [ final.null-db ];
+    };
   });
 }

shell.nix

@@ -0,0 +1 @@
+with import ./. { }; null-db.shell