oci: introduce WithSpecFromFile combinator

We introduce a WithSpecFromFile option combinator to allow creation
simpler creation of OCI specs from a file name. Often used as the first
option in a `SpecOpts` slice, it simplifies choosing between a local
file and the built-in default.

The code in `ctr run` has been updated to use the new option, with out
changing the order of operations or functionality present there.

Signed-off-by: Stephen Day <stephen.day@getcruise.com>

oci/spec_windows.go

@@ -22,8 +22,8 @@ import (
 	specs "github.com/opencontainers/runtime-spec/specs-go"
 )
 
-func createDefaultSpec(ctx context.Context, id string) (*Spec, error) {
-	return &Spec{
+func populateDefaultSpec(ctx context.Context, s *Spec, id string) error {
+	*s = Spec{
 		Version: specs.Version,
 		Root:    &specs.Root{},
 		Process: &specs.Process{
@@ -39,5 +39,6 @@ func createDefaultSpec(ctx context.Context, id string) (*Spec, error) {
 				AllowUnqualifiedDNSQuery: true,
 			},
 		},
-	}, nil
+	}
+	return nil
 }