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>

docker.go

@@ -63,6 +63,7 @@ type (
 		SecretFiles []string // Docker build secrets with file as source
 		AddHost     []string // Docker build add-host
 		Quiet       bool     // Docker build quiet
+		Platform    string   // Docker build platform
 	}
 
 	// Plugin defines the Docker plugin parameters.
@@ -324,6 +325,9 @@ func commandBuild(build Build) *exec.Cmd {
 	if build.Quiet {
 		args = append(args, "--quiet")
 	}
+	if build.Platform != "" {
+		args = append(args, "--platform", build.Platform)
+	}
 
 	if build.AutoLabel {
 		labelSchema := []string{