builder: set buildkit's exported product variable via PRODUCT

This introduces a PRODUCT environment variable that is used to set a constant
at dockerversion.ProductName.

That is then used to set BuildKit's ExportedProduct variable in order to show
useful error messages to users when a certain version of the product doesn't
support a BuildKit feature.

Signed-off-by: Tibor Vass <tibor@docker.com>
Upstream-commit: 195919d9d645aa4ab5680a2331c57ff33eb9e5d9
Component: engine

components/engine/cmd/dockerd/docker.go

@@ -10,6 +10,7 @@ import (
 	"github.com/docker/docker/dockerversion"
 	"github.com/docker/docker/pkg/reexec"
 	"github.com/docker/docker/pkg/term"
+	"github.com/moby/buildkit/util/apicaps"
 	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 )
@@ -42,6 +43,12 @@ func newDaemonCommand() *cobra.Command {
 	return cmd
 }
 
+func init() {
+	if dockerversion.ProductName != "" {
+		apicaps.ExportedProduct = dockerversion.ProductName
+	}
+}
+
 func main() {
 	if reexec.Init() {
 		return