.devcontainer.json: add Codespaces configuration

.devcontainer.json

@@ -0,0 +1,3 @@
+{
+  "image": "ghcr.io/zombiezen/codespaces-nix"
+}

shell.nix

@@ -0,0 +1,40 @@
+{ pkgs ? import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/7d0ed7f2e5aea07ab22ccb338d27fbe347ed2f11.tar.gz") {}
+, gitVersion ? "latest"
+}:
+
+let
+  defaultArgs = {
+    guiSupport = false;
+    sendEmailSupport = false;
+    svnSupport = false;
+    perlLibs = [pkgs.perlPackages.LWP pkgs.perlPackages.URI pkgs.perlPackages.TermReadKey];
+    smtpPerlLibs = [];
+  };
+  callPast = commit: relPath: args: pkgs.callPackage
+    ((fetchTarball "https://github.com/NixOS/nixpkgs/archive/${commit}.tar.gz") + "/" + relPath)
+    (defaultArgs // args);
+
+  gits = {
+    "2.17.1" = callPast
+      "9db1f486e15107e417b63119ad5e1917ee126599"
+      "pkgs/applications/version-management/git-and-tools/git"
+      {
+        stdenv = pkgs.stdenv // { inherit (pkgs) lib; };
+        python = pkgs.python3;
+      };
+    "2.25.1" = callPast
+      "b2a903a3e7ac9c038ed5f6a3ee744496622e0b65"
+      "pkgs/applications/version-management/git-and-tools/git"
+      {
+        stdenv = pkgs.stdenv // { inherit (pkgs) lib; };
+      };
+    latest = pkgs.git;
+  };
+in
+
+pkgs.mkShell {
+  packages = [
+    pkgs.go_1_20
+    (builtins.getAttr gitVersion gits)
+  ];
+}