Make chroot builds easier to set up

By default, we now include /bin/sh as a bind-mount of bash.

release.nix

@@ -93,6 +93,14 @@ let
           --sysconfdir=/etc
         '';
 
+        # Provide a default value for the ‘build-chroot-dirs’ setting
+        # that includes /bin/sh pointing to bash.
+        preHook = lib.optionalString stdenv.isLinux (
+          let sh = stdenv.shell; in
+          ''
+            NIX_CFLAGS_COMPILE+=" -DDEFAULT_CHROOT_DIRS=\"/bin/sh=${sh}:$(tr '\n' ':' < ${writeReferencesToFile sh})\""
+          '');
+
         enableParallelBuilding = true;
 
         makeFlags = "profiledir=$(out)/etc/profile.d";

src/libstore/globals.cc

@@ -48,6 +48,9 @@ Settings::Settings()
     useSubstitutes = true;
     buildUsersGroup = getuid() == 0 ? "nixbld" : "";
     useChroot = false;
+#ifdef DEFAULT_CHROOT_DIRS
+    dirsInChroot = tokenizeString<StringSet>(DEFAULT_CHROOT_DIRS, ":");
+#endif
     useSshSubstituter = true;
     impersonateLinux26 = false;
     keepLog = true;