Clean up/make consistent the APIs for registering core components, make Docker an optional component at compile time (#1111)

api/agent/drivers/docker/docker.go

@@ -646,3 +646,9 @@ func (w *waitResult) wait(ctx context.Context) (status string, err error) {
 }
 
 var _ drivers.Driver = &DockerDriver{}
+
+func init() {
+	drivers.Register("docker", func(config drivers.Config) (drivers.Driver, error) {
+		return NewDocker(config), nil
+	})
+}

api/agent/drivers/register.go

@@ -0,0 +1,26 @@
+package drivers
+
+import (
+	"fmt"
+	"github.com/sirupsen/logrus"
+)
+
+type DriverFunc func(config Config) (Driver, error)
+
+var drivers = make(map[string]DriverFunc)
+
+// Register adds  a container driver by name to this process
+func Register(name string, driverFunc DriverFunc) {
+	logrus.Infof("Registering container driver '%s'", name)
+	drivers[name] = driverFunc
+}
+
+// New Instantiates a driver by name
+func New(driverName string, config Config) (Driver, error) {
+	driverFunc, ok := drivers[driverName]
+
+	if !ok {
+		return nil, fmt.Errorf("agent driver \"%s\" is not registered", driverName)
+	}
+	return driverFunc(config)
+}