Ensure passwd is available in the build environment

We can likely create our user from the outside as an even better
solution, but for now just depend on passwd to fix the immediate issue.

Resolves: #37

debspawn/osbase.py

@@ -492,7 +492,7 @@ def _create_internal(
         if with_init:
             print('Includes init: yes')
 
-        include_pkgs = ['python3-minimal', 'eatmydata']
+        include_pkgs = ['passwd', 'python3-minimal', 'eatmydata']
         if with_init:
             include_pkgs.append('systemd-sysv')
         cmd = [bootstrap_tool_exe, '--arch={}'.format(self.arch), '--include=' + ','.join(include_pkgs)]