update docker sample by caching dependencies

Signed-off-by: Jérémie Drouet <jeremie.drouet@gmail.com>

docker_sample/src/main.rs

@@ -24,3 +24,11 @@ async fn main() -> std::io::Result<()> {
         .run()
         .await
 }
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn sample_test() {
+        assert!(true);
+    }
+}