add nix-shell for local development

default.nix

@@ -1,7 +1,6 @@
 { pkgs ? import <nixpkgs> { } }:
 let
-  f = (path: type: ! builtins.elem (baseNameOf path) [ ".git" "node_modules" "result" ]);
-  src = builtins.filterSource f ./.;
+  src = import ./source.nix;
 in
 pkgs.mastodon.override {
   pname = "mastodon-queer-af";

shell.nix

@@ -0,0 +1,24 @@
+{ pkgs ? import <nixpkgs> { } }:
+let
+  src = import ./source.nix;
+  gems = pkgs.bundlerEnv {
+    name = "mastodon-gems";
+    gemdir = ./.;
+  };
+  yarns = pkgs.mkYarnPackage {
+    pname = "mastodon-yarns";
+    yarnNix = ./yarn.nix;
+    packageJSON = ./package.json;
+    version = "0.0.0";
+    inherit src;
+  };
+in
+pkgs.mkShell {
+  packages = [ gems gems.wrappedRuby pkgs.yarn pkgs.nodejs pkgs.foreman ];
+
+  RAILS_ENV = "development";
+
+  # The rails bits default to this, but the node bits don't, which cause weird
+  # errors trying to run the streaming component without sourcing a .envrc.
+  DB_HOST = "/var/run/postgresql";
+}

source.nix

@@ -0,0 +1,4 @@
+let
+  f = (path: type: ! builtins.elem (baseNameOf path) [ ".git" "node_modules" "result" ]);
+in
+builtins.filterSource f ./.