containerboot: Add TS_ACCEPT_ROUTES (#10176)

Fixes tailscale/corp#15596

Signed-off-by: Claire Wang <claire@tailscale.com>

cmd/containerboot/main_test.go

@@ -575,6 +575,22 @@ func TestContainerBoot(t *testing.T) {
 				},
 			},
 		},
+		{
+			Name: "extra_args_accept_routes",
+			Env: map[string]string{
+				"TS_EXTRA_ARGS": "--accept-routes",
+			},
+			Phases: []phase{
+				{
+					WantCmds: []string{
+						"/usr/bin/tailscaled --socket=/tmp/tailscaled.sock --state=mem: --statedir=/tmp --tun=userspace-networking",
+						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --accept-routes",
+					},
+				}, {
+					Notify: runningNotify,
+				},
+			},
+		},
 		{
 			Name: "hostname",
 			Env: map[string]string{