Extract Docker RUN commands into shell scripts for better maintainability

Co-authored-by: johndoe6345789 <224850594+johndoe6345789@users.noreply.github.com>

scripts/docker-image-install-tools.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+# Install required tools for building UEFI-bootable SparkOS image
+
+set -e
+
+apt-get update
+apt-get install -y \
+    gcc \
+    make \
+    dosfstools \
+    mtools \
+    e2fsprogs \
+    parted \
+    gdisk \
+    grub-efi-amd64-bin \
+    grub-common \
+    wget \
+    busybox-static \
+    kmod
+rm -rf /var/lib/apt/lists/*