nixos/lib/make-disk-image: introduce noNixStore option

Sometimes, we want to use the host nix store for everything because it's
more convenient.

To do so, we need to "not install" the nix store in the target disk
image.

We trick nixos-install by mount binding the host nix store into the
$root/nix folder and letting Nix do the idempotency thing.

nixos/lib/make-disk-image.nix

@@ -162,6 +162,12 @@ To solve this, you can run `fdisk -l $image` and generate `dd if=$image of=$imag
   # `installBootLoader` and `configFile`.
   onlyNixStore ? false
 
+, # Ensure that no "new" Nix store is written to the root of the image
+  # by symlinking the host Nix store.
+  # This saves a lot of disk space if you don't need an isolated Nix store.
+  # Incompatible with `additionalPaths`
+  noNixStore ? true
+
 , name ? "nixos-disk-image"
 
 , # Disk image format, one of qcow2, qcow2-compressed, vdi, vpc, raw.
@@ -200,6 +206,7 @@ assert (lib.assertMsg (partitionTableType != "none" -> fsType == "ext4") "to pro
 assert (lib.assertMsg (touchEFIVars -> partitionTableType == "hybrid" || partitionTableType == "efi" || partitionTableType == "legacy+gpt") "EFI variables can be used only with a partition table of type: hybrid, efi or legacy+gpt.");
 # If only Nix store image, then: contents must be empty, configFile must be unset, and we should no install bootloader.
 assert (lib.assertMsg (onlyNixStore -> contents == [] && configFile == null && !installBootLoader) "In a only Nix store image, the contents must be empty, no configuration must be provided and no bootloader should be installed.");
+assert (lib.assertMsg (noNixStore -> additionalPaths == []) "Without a Nix store in the rootfs, additional paths cannot be copied otherwise you would end up again with a Nix store.");
 # Either both or none of {user,group} need to be set
 assert (lib.assertMsg (lib.all
          (attrs: ((attrs.user  or null) == null)
@@ -422,6 +429,15 @@ let format' = format; in let
     export NIX_STATE_DIR=$TMPDIR/state
     nix-store --load-db < ${closureInfo}/registration
 
+    ${optionalString noNixStore ''
+      # Ensure host Nix store is available so that `nixos-install` performs
+      # no extra operations.
+      mkdir -p $root/nix
+      # TODO: this needs to be made persistent or you need to run the nixos-install inside the namespace.
+      # then, we need to unmount.
+      unshare --mount --map-root-user mount --rbind ${builtins.storeDir} $root/nix
+    ''}
+
     chmod 755 "$TMPDIR"
     echo "running nixos-install..."
     nixos-install --root $root --no-bootloader --no-root-passwd \