Mention requirement to use checkout

[kalikiana]

I think the usage example should include the checkout action:

uses: actions/checkout@v2

Because otherwise you get this error trying to use hadolint:

hadolint: ./container/webui/Dockerfile: openBinaryFile: does not exist (No such file or directory)

[brpaz]

Well, I can add that, I thought it was pretty obvious, since if you don´t checkout your code, how hadolint will have anything to analyze?

[lorenzo]

I think it would be idea to have a full copy-pasteable example @brpaz

[kalikiana]

Yes. If you provide an example like that I kind of expect it to work as-is.

[lorenzo]

@kalikiana would you like to contribute the example to the readme?