docker: Update debootstrap script after Debian migration from Alioth to Salsa

This silents the following warning:

  Cloning into './debootstrap.git'...
  warning: redirecting to https://salsa.debian.org/installer-team/debootstrap.git/

See https://lists.debian.org/debian-devel-announce/2018/01/msg00004.html



Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

tests/docker/dockerfiles/debian-bootstrap.pre

@@ -62,7 +62,7 @@ if [ -z $DEBOOTSTRAP_DIR ]; then
         NEED_DEBOOTSTRAP=true
     fi
     if $NEED_DEBOOTSTRAP; then
-        DEBOOTSTRAP_SOURCE=https://anonscm.debian.org/git/d-i/debootstrap.git
+        DEBOOTSTRAP_SOURCE=https://salsa.debian.org/installer-team/debootstrap.git
         git clone ${DEBOOTSTRAP_SOURCE} ./debootstrap.git
         export DEBOOTSTRAP_DIR=./debootstrap.git
         DEBOOTSTRAP=./debootstrap.git/debootstrap