Excluded docker storage from the test machine chroot

Copying /var/lib/docker-storage.btrfs into the chroot filled the disk on
build hosts running docker.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>
(cherry picked from commit 37f32ae)

Author: larsewi

build-scripts/functions

@@ -459,6 +459,7 @@ EOF
 - /run/*
 + /run
 - /var/lib/lxcfs
+- /var/lib/docker*
 EOF
         ;;
     esac