cmd/create, pkg/skopeo: Make the 'skopeo inspect' invocation cancellable

A subsequent commit will use this to ensure that the user can still
interact with the image download prompt while 'skopeo inspect' fetches
the image size from the remote registry.  Initially, the prompt will be
shown without the image size.  If the user responds before the size is
fetched, then the pending 'skopeo inspect' will be cancelled.

#752
#1263

src/cmd/create.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
@@ -576,7 +577,8 @@ func getFullyQualifiedImageFromRepoTags(image string) (string, error) {
 }
 
 func getImageSizeFromRegistry(imageFull string) (string, error) {
-	image, err := skopeo.Inspect(imageFull)
+	ctx := context.Background()
+	image, err := skopeo.Inspect(ctx, imageFull)
 	if err != nil {
 		return "", err
 	}

src/pkg/skopeo/skopeo.go

@@ -18,6 +18,7 @@ package skopeo
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 
 	"github.com/containers/toolbox/pkg/shell"
@@ -30,13 +31,13 @@ type Image struct {
 	LayersData []Layer
 }
 
-func Inspect(target string) (*Image, error) {
+func Inspect(ctx context.Context, target string) (*Image, error) {
 	var stdout bytes.Buffer
 
 	targetWithTransport := "docker://" + target
 	args := []string{"inspect", "--format", "json", targetWithTransport}
 
-	if err := shell.Run("skopeo", nil, &stdout, nil, args...); err != nil {
+	if err := shell.RunContext(ctx, "skopeo", nil, &stdout, nil, args...); err != nil {
 		return nil, err
 	}