The previous build workflow used emulation to build the Docker image,
which results in a somewhat complicated push-by-digest and merge
workflow to create a multi-platform image.

This commit changes the Docker build to use cross-compilation instead,
resulting in a faster and more straightforward build.

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

* Rename /site to /project
* Add ldflags
* Add go and node to the default image
* Add Dart Sass to the default image
* Build the extended version by default
* Add "npm i" install support with custom entry script override
* Adjust cache logic to speed up CGO rebuilds

Closes #12920
See #12885