Adding file to packages

Build a clean image, but build.sh complains that 'file' is missing from the system.

Dockerfile

@@ -6,7 +6,7 @@ RUN apt-get -y update && \
     apt-get -y install \
         git vim parted \
         quilt realpath qemu-user-static debootstrap zerofree pxz zip dosfstools \
-        bsdtar libcap2-bin rsync grep udev xz-utils curl xxd \
+        bsdtar libcap2-bin rsync grep udev xz-utils curl xxd file \
     && rm -rf /var/lib/apt/lists/*
 
 COPY . /pi-gen/