automatically start/upgrade buildkitd

- Automatically start a buildkit daemon if no BUILDKIT_HOST is
  provided (and if not already started)
- Customization of BUILDKIT_HOST is still possible, just like before
- Automatically upgrade the managed daemon to the version used by dagger
  if necessary
- Add CI test to make sure the managed buildkit and the vendored
  buildkit versions match

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

dagger/client.go

@@ -22,15 +22,12 @@ import (
 	bkgw "github.com/moby/buildkit/frontend/gateway/client"
 
 	// docker output
+	"dagger.io/go/pkg/buildkitd"
 	"dagger.io/go/pkg/progressui"
 
 	"dagger.io/go/dagger/compiler"
 )
 
-const (
-	defaultBuildkitHost = "docker-container://buildkitd"
-)
-
 // A dagger client
 type Client struct {
 	c *bk.Client
@@ -41,7 +38,12 @@ func NewClient(ctx context.Context, host string) (*Client, error) {
 		host = os.Getenv("BUILDKIT_HOST")
 	}
 	if host == "" {
-		host = defaultBuildkitHost
+		h, err := buildkitd.Start(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		host = h
 	}
 	c, err := bk.New(ctx, host)
 	if err != nil {