docker: Don't mount ccache db if NOUSER=1

With NOUSER=1 the container runs code as root, which may create
privileged files that will not be be accssible next time. Skip ccache
dir mount in this case.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170925075458.18047-1-famz@redhat.com>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
