nfs-utils initscript uses killall option not available in Alpine
killall -q -HUP rpc.idmapd
attempts to pass the signal as a bare option, but the version of killall on Alpine doesn't support that way of specifying the signal, so it just does this:
# rc-service nfs start * Starting rpcbind ... [ ok ] * Starting NFS statd ... [ ok ] Usage: killall [OPTION]... [--] NAME... ---snip---
I suspect that fixing this is as easy as using
killall -q -s HUP rpc.idmapd
but I'm no expert (I'm just a user trying to run an NFS server on Alpine).