Simplify dev container configuration

Author: daohoangson

.devcontainer/Dockerfile

@@ -1,3 +1,3 @@
 {
-  "dockerFile": "Dockerfile"
+  "image": "cirrusci/flutter:stable"
 }

.devcontainer/build.sh

@@ -17,6 +17,4 @@ This repo contains the source code for everything `HtmlWidget`-related.
 | [Demo app](./demo_app/)                                            | https://demo.fwfh.dev, [supported/tags.html](https://demo.fwfh.dev/supported/tags.html)                                                  |
 | Special [dartpad](https://dartpad.dev) with our packages           | https://try.fwfh.dev                                                                                                                     |
 
-[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/daohoangson/flutter_widget_from_html)
-
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