Dockerfile: Add fdisk

We had previously gotten this package through a chain of dependencies
with guestfs-tools. Now that we no longer install that package, install
fdisk (for sfdisk) directly.

Fixes: eb1b90ec57 ("Dockerfile: Update to drop virt-make-fs packages")
Signed-off-by: Tom Rini <trini@konsulko.com>

tools/docker/Dockerfile

@@ -78,6 +78,7 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 	exfatprogs \
 	expect \
 	fakeroot \
+	fdisk \
 	flex \
 	gawk \
 	gdisk \