docker: silence debootstrap when --quiet is given

If we silence docker when --quiet is given, we should also silence the
.pre script (i.e. debootstrap).

Only discards stdout, so some diagnostics (e.g. from git clone) are
still printed. Most of the verbose output is gone however and this way
we still have a chance to see error messages.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1473192351-601-9-git-send-email-silbe@linux.vnet.ibm.com>
Signed-off-by: Fam Zheng <famz@redhat.com>