driver: basic types startpoint

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

driver/manager.go

@@ -0,0 +1,56 @@
+package driver
+
+import (
+	"context"
+	"sort"
+
+	dockerclient "github.com/docker/docker/client"
+	"github.com/pkg/errors"
+)
+
+type Factory interface {
+	Name() string
+	Usage() string
+	Priority() int // take initConfig?
+	New(ctx context.Context, cfg InitConfig) (Driver, error)
+}
+
+type BuildkitConfig struct {
+	// Entitlements []string
+	// Rootless bool
+}
+
+type InitConfig struct {
+	// This object needs updates to be generic for different drivers
+	Name           string
+	DockerAPI      dockerclient.APIClient
+	BuildkitConfig BuildkitConfig
+	Meta           map[string]interface{}
+}
+
+var drivers map[string]Factory
+
+func Register(f Factory) {
+	if drivers == nil {
+		drivers = map[string]Factory{}
+	}
+	drivers[f.Name()] = f
+}
+
+func GetDefaultDriver() (Factory, error) {
+	if len(drivers) == 0 {
+		return nil, errors.Errorf("no drivers available")
+	}
+	type p struct {
+		f        Factory
+		priority int
+	}
+	dd := make([]p, 0, len(drivers))
+	for _, f := range drivers {
+		dd = append(dd, p{f: f, priority: f.Priority()})
+	}
+	sort.Slice(dd, func(i, j int) bool {
+		return dd[i].priority < dd[j].priority
+	})
+	return dd[0].f, nil
+}