Fix docker build with debian bullseye image

- Fix missing sfdisk which is not included anymore in bullseye container
- Fix problem with binfmt_misc not being mounted inside docker although present on host

Dockerfile

@@ -8,7 +8,7 @@ RUN apt-get -y update && \
         git vim parted \
         quilt coreutils qemu-user-static debootstrap zerofree zip dosfstools \
         libarchive-tools libcap2-bin rsync grep udev xz-utils curl xxd file kmod bc\
-        binfmt-support ca-certificates qemu-utils kpartx \
+        binfmt-support ca-certificates qemu-utils kpartx fdisk \
     && rm -rf /var/lib/apt/lists/*
 
 COPY . /pi-gen/