Docker: Add quilt to kas-isar

Will become a hard dependency when switching the (currently broken)
patch class of Isar to one derived from OE. Prepare for that.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile.isar

@@ -8,7 +8,7 @@ RUN apt-get update && \
             autoconf automake gdisk libtool bash-completion \
             sudo grub2 grub-efi-amd64-bin grub-efi-ia32-bin \
             reprepro python3 binfmt-support e2fsprogs \
-            multistrap qemu-user-static debootstrap && \
+            multistrap qemu-user-static debootstrap quilt && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp*