Ensure NFS server run in alpine:3.12
The latest update to 3.13 broke the NFS server. The,
capsh --print | grep -Eq "^Current: = .*,?{1}(,|)"
Check for cap_sys_admin works when running privileged alpine:3.12
containers, but does not when running with alpine:3.13 containers.
To avoid investigating this, pin to a static release so this doesn't bite again.