Improvement: Build Default Image using a GitHub Action

[theGOTOguy]

For my own custom build utilizing Pi-Gen, I have written a GitHub action that automatically builds the image, which is then available to download directly through GitHub.

This would:

1. Automatically confirm on each push that the build process isn't broken.
2. Provide users with a ready-built Raspbian image with the most common settings for download and testing.
3. Serve as a blueprint for others who might like to build custom images based on Raspbian.

Please feel free to use my existing, open-source action above at a starting point. Or, if there is interest from the devs, I will be happy to contribute a PR for you that would build a Raspbian image with the most common settings.

[XECDesign]

That's great, thanks!

We use Gitlab CI for our official builds, but having a github workflow as an example for others would be useful.

I'd merge a PR that builds the default images without customisation.

I'm guessing Github doesn't provide enough disk space to build non-lite images, but that problem should be solved when I add COW support for stage copies.

[jedahan]

Here's another example workflow and the build-image.sh that runs inside the build step. Couldn't have gotten this far without @theGOTOguy work, so wanted to reciprocate.