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>

cmd/ctr/commands/run/run.go

@@ -19,9 +19,7 @@ package run
 import (
 	gocontext "context"
 	"encoding/csv"
-	"encoding/json"
 	"fmt"
-	"io/ioutil"
 	"strings"
 
 	"github.com/containerd/console"
@@ -37,17 +35,6 @@ import (
 	"github.com/urfave/cli"
 )
 
-func loadSpec(path string, s *specs.Spec) error {
-	raw, err := ioutil.ReadFile(path)
-	if err != nil {
-		return errors.New("cannot load spec config file")
-	}
-	if err := json.Unmarshal(raw, s); err != nil {
-		return errors.Errorf("decoding spec config file failed, current supported OCI runtime-spec : v%s", specs.Version)
-	}
-	return nil
-}
-
 func withMounts(context *cli.Context) oci.SpecOpts {
 	return func(ctx gocontext.Context, client oci.Client, container *containers.Container, s *specs.Spec) error {
 		mounts := make([]specs.Mount, 0)