Implement new driver-opt: default-load

This eases build driver migrations, as it allows aligning the default behavior.
See also https://docs.docker.com/build/drivers/

Signed-off-by: Niklas Gehlen <niklas@namespacelabs.com>

driver/docker-container/driver.go

@@ -56,6 +56,7 @@ type Driver struct {
 	cgroupParent  string
 	restartPolicy container.RestartPolicy
 	env           []string
+	defaultLoad   bool
 }
 
 func (d *Driver) IsMobyDriver() bool {
@@ -423,6 +424,7 @@ func (d *Driver) Features(ctx context.Context) map[driver.Feature]bool {
 		driver.DockerExporter: true,
 		driver.CacheExport:    true,
 		driver.MultiPlatform:  true,
+		driver.DefaultLoad:    d.defaultLoad,
 	}
 }
 

driver/docker-container/factory.go

@@ -94,6 +94,11 @@ func (f *factory) New(ctx context.Context, cfg driver.InitConfig) (driver.Driver
 			if err != nil {
 				return nil, err
 			}
+		case k == "default-load":
+			d.defaultLoad, err = strconv.ParseBool(v)
+			if err != nil {
+				return nil, err
+			}
 		case strings.HasPrefix(k, "env."):
 			envName := strings.TrimPrefix(k, "env.")
 			if envName == "" {