Add Tailscale unstable channel and repo HEAD to integration tests

In preparation for the implementation of the new TS2021 protocol (Tailscale control protocol v2) we are expanding the test infrastructure

integration_embedded_derp_test.go

@@ -245,16 +245,8 @@ func (s *IntegrationDERPTestSuite) Join(
 
 func (s *IntegrationDERPTestSuite) tailscaleContainer(identifier, version string, network dockertest.Network,
 ) (string, *dockertest.Resource) {
-	tailscaleBuildOptions := &dockertest.BuildOptions{
-		Dockerfile: "Dockerfile.tailscale",
-		ContextDir: ".",
-		BuildArgs: []docker.BuildArg{
-			{
-				Name:  "TAILSCALE_VERSION",
-				Value: version,
-			},
-		},
-	}
+	tailscaleBuildOptions := getDockerBuildOptions(version)
+
 	hostname := fmt.Sprintf(
 		"tailscale-%s-%s",
 		strings.Replace(version, ".", "-", -1),