[build]: Update comments on build_debian.sh (sonic-net#3533)

Author: qiluo-msft

build_debian.sh

@@ -170,7 +170,7 @@ sudo chmod +x $FILESYSTEM_ROOT/etc/initramfs-tools/scripts/init-premount/resize-
 
 ## Hook into initramfs: after partition mount and loop file mount
 ## 1. Prepare layered file system
-## 2. Bind-mount docker working directory (docker aufs cannot work over aufs rootfs)
+## 2. Bind-mount docker working directory (docker overlay storage cannot work over overlay rootfs)
 sudo cp files/initramfs-tools/union-mount $FILESYSTEM_ROOT/etc/initramfs-tools/scripts/init-bottom/union-mount
 sudo chmod +x $FILESYSTEM_ROOT/etc/initramfs-tools/scripts/init-bottom/union-mount
 sudo cp files/initramfs-tools/varlog $FILESYSTEM_ROOT/etc/initramfs-tools/scripts/init-bottom/varlog
@@ -206,7 +206,7 @@ sudo LANG=C chroot $FILESYSTEM_ROOT apt-get update
 sudo LANG=C chroot $FILESYSTEM_ROOT apt-get -y install docker-ce=${DOCKER_VERSION}
 sudo LANG=C chroot $FILESYSTEM_ROOT apt-get -y remove software-properties-common gnupg2
 
-## Add docker config drop-in to select aufs, otherwise it may select other storage driver
+## Add docker config drop-in to specify dockerd command line
 sudo mkdir -p $FILESYSTEM_ROOT/etc/systemd/system/docker.service.d/
 ## Note: $_ means last argument of last command
 sudo cp files/docker/docker.service.conf $_