docker: build debootstrap after cloning

When using the git version of debootstrap (because no usable version
of debootstrap was installed on the host), we need to run 'make' so
that devices.tar.gz gets built. Otherwise the first debootstrap stage
will fail without printing any error message.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Message-Id: <1473192351-601-8-git-send-email-silbe@linux.vnet.ibm.com>
Signed-off-by: Fam Zheng <famz@redhat.com>