[RFE] docs: Add documentation on how to build Flatcar in the fastest and most energy efficient way possible

[ader1990]

Current situation

Flatcar full build - starting from a clean build environment and to a fully fledged bootable image takes around 2 hours on a modern build environment. On a state of the art build environment, the build time takes 1.5 hours.

Flatcar is a Gentoo derivative, source based operating system. Its immutability is important in this regard too, as one cannot build just one subset of Flatcar packages, all the packages need to be built to create a production image.

Impact

First time Flatcar builders, engineers and community do not have a clear view on build times, hardware needed or software setups for the fastest build.

Ideal future situation

Document the best ways to achieve the fastest possible builds in terms of hardware, software and workflow.

Implementation options

Create a pull request to document the information.