This requires switching to the dosfstools from alpine:edge since neither the
busybox nor alpine:3.5 dosfstools supports the -C option (in fact alpine:3.5
only has mkfs.fat and not mkfs.vfat).
The 511k slack seems like a lot to me, but 256k was somehow not enough.
Fixes#1304.
Signed-off-by: Ian Campbell <ian.campbell@docker.com>
Note that the EFI ISO is not yet automatically sized, and the
kernel command lines are currently hard coded in the builders.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
