cli/command/image: move build-context detection to build

Removes direct imports of github.com/docker/docker/builder in
the image package, to be moved later.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/build/context_detect.go

@@ -0,0 +1,39 @@
+package build
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/docker/docker/builder/remotecontext/urlutil"
+)
+
+// ContextType describes the type (source) of build-context specified.
+type ContextType string
+
+const (
+	ContextTypeStdin  ContextType = "stdin"  // ContextTypeStdin indicates that the build-context is a TAR archive passed through STDIN.
+	ContextTypeLocal  ContextType = "local"  // ContextTypeLocal indicates that the build-context is a local directory.
+	ContextTypeRemote ContextType = "remote" // ContextTypeRemote indicates that the build-context is a remote URL.
+	ContextTypeGit    ContextType = "git"    // ContextTypeGit indicates that the build-context is a GIT URL.
+)
+
+// DetectContextType detects the type (source) of the build-context.
+func DetectContextType(specifiedContext string) (ContextType, error) {
+	switch {
+	case specifiedContext == "-":
+		return ContextTypeStdin, nil
+	case isLocalDir(specifiedContext):
+		return ContextTypeLocal, nil
+	case urlutil.IsGitURL(specifiedContext):
+		return ContextTypeGit, nil
+	case urlutil.IsURL(specifiedContext):
+		return ContextTypeRemote, nil
+	default:
+		return "", fmt.Errorf("unable to prepare context: path %q not found", specifiedContext)
+	}
+}
+
+func isLocalDir(c string) bool {
+	_, err := os.Stat(c)
+	return err == nil
+}