build: fix sharedkey computation for local context

When LocalDirs were changed to LocalMounts, this broke the
sharedKey computation that was based on the context directory
path. SharedKey defines if directory is valid candidate for
incremental context transfer and if not set properly then
different directories do metadata-based transfers to same destination.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/opt.go

@@ -272,11 +272,9 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt Op
 	}
 	defers = append(defers, releaseLoad)
 
-	if sharedKey := so.LocalDirs["context"]; sharedKey != "" {
-		if p, err := filepath.Abs(sharedKey); err == nil {
-			sharedKey = filepath.Base(p)
-		}
-		so.SharedKey = sharedKey + ":" + confutil.TryNodeIdentifier(configDir)
+	// add node identifier to shared key if one was specified
+	if so.SharedKey != "" {
+		so.SharedKey += ":" + confutil.TryNodeIdentifier(configDir)
 	}
 
 	if opt.Pull {
@@ -416,6 +414,11 @@ func loadInputs(ctx context.Context, d *driver.DriverHandle, inp Inputs, addVCSL
 		if err := setLocalMount("context", inp.ContextPath, target, addVCSLocalDir); err != nil {
 			return nil, err
 		}
+		sharedKey := inp.ContextPath
+		if p, err := filepath.Abs(sharedKey); err == nil {
+			sharedKey = filepath.Base(p)
+		}
+		target.SharedKey = sharedKey
 		switch inp.DockerfilePath {
 		case "-":
 			dockerfileReader = inp.InStream