Alpine install: choose --target for grub and cleanup the gap

To avoid this error:

nb2:~nico/vcs/ungleich-tools# sudo chroot /tmp/tmp.U3RiZH2Nu9 /bin/sh -c 'grub-install --target=i386-pc /dev/sda'
Installing for i386-pc platform.
grub-install: warning: Attempting to install GRUB to a disk with multiple partition labels.  This is not supported yet..
grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.

See also:
https://askubuntu.com/questions/666527/install-grub-claims-that-i-have-multiple-partiton-labels-and-that-embedding-is

alpine-install-on-disk.sh

@@ -32,6 +32,10 @@ run_root () {
 
 wget -c "$rootfs_url" -O "$IMAGE"
 
+# Clean the first 2M - getting rid of old things
+# in the gap and also the paritition table
+dd if=/dev/zero of=${DISK} bs=1M count=2
+
 # Partition disk with 1 Linux partition
 sudo sfdisk "$DISK" <<EOF
 label: dos
@@ -86,7 +90,7 @@ for dir in dev proc sys; do
 done
 
 run_root apk add grub-bios
-run_root grub-install ${DISK}
+run_root grub-install --target=i386-pc ${DISK}
 
 # Cleanup
 run_root rm -f /etc/resolv.conf