Install gdisk/dosfstools on nodepool-builder images

gdisk includes sgdisk which is used to create GPT partitions on disks.
EFI partitions are vfat so need dosfstools.  This is used by DIB when
creating images with a EFI setup as with arm64 image builds.

Change-Id: I57891d6890a3db6acb42c149c3a05ab25f423385

Dockerfile

@@ -77,6 +77,8 @@ RUN \
   && apt-get install -y \
       curl \
       debian-keyring \
+      dosfstools \
+      gdisk \
       git \
       kpartx \
       qemu-utils \