feat: add support for sysctls compose key (#239)

* feat: add support for `sysctls` compose key

* lint

---------

Co-authored-by: Pasha Sviderski <me@psviderski.name>

pkg/client/compose/service_test.go

@@ -125,6 +125,9 @@ func TestServiceSpecFromCompose(t *testing.T) {
 							Memory:            100 * units.MiB,
 							MemoryReservation: 50 * units.MiB,
 						},
+						Sysctls: map[string]string{
+							"net.ipv4.ip_forward": "1",
+						},
 						User: "nginx:nginx",
 						VolumeMounts: []api.VolumeMount{
 							{