build: send individual shared keys for local build contexts

Allows avoiding the case where two separate builds using
same context name would use the same destination directory
for upload.

Requires updated frontend to handle these new keys.

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

build/opt.go

@@ -546,8 +546,14 @@ func toSolveOpt(ctx context.Context, node builder.Node, multiDriver bool, opt *O
 	}
 
 	// add node identifier to shared key if one was specified
+	nodeID := cfg.TryNodeIdentifier()
 	if so.SharedKey != "" {
-		so.SharedKey += ":" + cfg.TryNodeIdentifier()
+		so.SharedKey += ":" + nodeID
+	}
+	for k, v := range so.FrontendAttrs {
+		if strings.HasPrefix(k, "sharedkey:localdir:") {
+			so.FrontendAttrs[k] = v + ":" + nodeID
+		}
 	}
 
 	if opt.Pull {
@@ -855,6 +861,13 @@ func loadInputs(ctx context.Context, d *driver.DriverHandle, inp *Inputs, pw pro
 			return nil, err
 		}
 		target.FrontendAttrs["context:"+k] = "local:" + localName
+		sharedKey := v.Path
+		if p, err := filepath.Abs(sharedKey); err == nil {
+			sharedKey = filepath.Base(p)
+		} else {
+			sharedKey = filepath.Base(sharedKey)
+		}
+		target.FrontendAttrs["sharedkey:localdir:"+k] = sharedKey
 	}
 
 	release := func() {