Add support for docker --platform flag

Add a --platform flag so that the user can specify a value for the
corresponding flag passed to docker. This is useful in the context of
multi-arch builds and buildkit (DOCKER_BUILDKIT=1 in the environment).

Signed-off-by: Marcelo E. Magallon <marcelo.magallon@grafana.com>

cmd/drone-docker/main.go

@@ -269,6 +269,11 @@ func main() {
 			Usage:  "card path location to write to",
 			EnvVar: "DRONE_CARD_PATH",
 		},
+		cli.StringFlag{
+			Name:   "platform",
+			Usage:  "platform value to pass to docker",
+			EnvVar: "PLUGIN_PLATFORM",
+		},
 	}
 
 	if err := app.Run(os.Args); err != nil {
@@ -312,6 +317,7 @@ func run(c *cli.Context) error {
 			SecretFiles: c.StringSlice("secrets-from-file"),
 			AddHost:     c.StringSlice("add-host"),
 			Quiet:       c.Bool("quiet"),
+			Platform:    c.String("platform"),
 		},
 		Daemon: docker.Daemon{
 			Registry:      c.String("docker.registry"),