chore(docs): Move mdBook derivation to docs/default.nix

default.nix

@@ -25,32 +25,11 @@ rec {
   # data dependencies.
   nixery-server = callPackage ./server {};
 
-  # Nix expression (unimported!) which is used by Nixery to build
-  # container images.
-  nixery-builder = ./build-registry-image.nix;
-
-  # nixpkgs currently has an old version of mdBook. A new version is
-  # built here, but eventually the update will be upstreamed
-  # (nixpkgs#65890)
-  mdbook = rustPlatform.buildRustPackage rec {
-    name = "mdbook-${version}";
-    version = "0.3.1";
-    doCheck = false;
-
-    src = fetchFromGitHub {
-      owner = "rust-lang-nursery";
-      repo = "mdBook";
-      rev = "v${version}";
-      sha256 = "0py69267jbs6b7zw191hcs011cm1v58jz8mglqx3ajkffdfl3ghw";
-    };
-
-    cargoSha256 = "0qwhc42a86jpvjcaysmfcw8kmwa150lmz01flmlg74g6qnimff5m";
-  };
 
   # Use mdBook to build a static asset page which Nixery can then
   # serve. This is primarily used for the public instance at
   # nixery.dev.
-  nixery-book = callPackage ./docs { inherit mdbook; };
+  nixery-book = callPackage ./docs {};
 
   # Wrapper script running the Nixery server with the above two data
   # dependencies configured.

docs/default.nix

@@ -18,9 +18,27 @@
 # Some of the documentation is pulled in and included from other
 # sources.
 
-{ fetchFromGitHub, mdbook, runCommand }:
+{ fetchFromGitHub, mdbook, runCommand, rustPlatform }:
 
 let
+  # nixpkgs currently has an old version of mdBook. A new version is
+  # built here, but eventually the update will be upstreamed
+  # (nixpkgs#65890)
+  mdbook = rustPlatform.buildRustPackage rec {
+    name = "mdbook-${version}";
+    version = "0.3.1";
+    doCheck = false;
+
+    src = fetchFromGitHub {
+      owner = "rust-lang-nursery";
+      repo = "mdBook";
+      rev = "v${version}";
+      sha256 = "0py69267jbs6b7zw191hcs011cm1v58jz8mglqx3ajkffdfl3ghw";
+    };
+
+    cargoSha256 = "0qwhc42a86jpvjcaysmfcw8kmwa150lmz01flmlg74g6qnimff5m";
+  };
+
   nix-1p = fetchFromGitHub {
     owner  = "tazjin";
     repo   = "nix-1p";