Always use our dpkg wrapper when mastering an image.

src/package_scripts/base-passwd.postinst

Index: src/package_scripts/base-passwd.postinst
diff --git a/src/package_scripts/base-passwd.postinst b/src/package_scripts/base-passwd.postinst
new file mode 100755
index 0000000000000000000000000000000000000000..f94639a375050a70c77baa6455996debd7e092c1
--- /dev/null
+++ b/src/package_scripts/base-passwd.postinst
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# base-passwd
+
+cp -a "${ROOT}/usr/share/base-passwd/passwd.master" \
+  "${ROOT}/etc/passwd"
+cp -a "${ROOT}/usr/share/base-passwd/group.master" \
+  "${ROOT}/etc/group"