Otherwise the suite (-s) option is rewritten to a non-existing
DEBIAN_STRETCH class which lacks the corresponding apt sources.list
file, resulting in no Debian mirror available in the chroot.
--- /dev/null
+# official debian repository:
+ deb http://ftp.debian.org/debian/ stretch main contrib non-free
+ deb-src http://ftp.debian.org/debian/ stretch main contrib non-free
+
+# security updates:
+ deb http://security.debian.org/ stretch/updates main contrib
+ deb-src http://security.debian.org/ stretch/updates main contrib
+
+# backports:
+ deb http://ftp.debian.org/debian/ stretch-backports main contrib non-free
+ deb-src http://ftp.debian.org/debian/ stretch-backports main contrib non-free