Restructure Docker image from scratch

[mottljan]

Basic info is provided in the README, CHANGELOG and even comments in the Dockerfile. What goes beyond the image:

• Since I changed user to nonroot, we will need to update Gradle caching on CI as well. I have a proposal how to do this using a pure Docker way, which IMO should work as expected, while removing that CI caching logic that nobody wants to touch :D I analyzed it and I believe it is useless and actually does not do anything useful. I will need at least read access to our puppet config or we can take a look together. I wanna at least discuss it for sure.
• Additionally, we will need to cache build-tools and platforms on runners as well. The same way as we should Gradle/deps. I removed all those versions from the image, because once there is an update and particular project updates compileSdk and/or AGP, they will never use the ones in the image and will download it in every single job (I verified this on CI). So since this is semantically the same as having a custom Gradle version and dependencies per project, we should treat it the same way, i.e. let the project resolve it itself but cache it for future usages / other projects.

If you have any additional questions, feel free to ask :)

yeah and the image size was reduced from over 16 GB to just 1,35 GB. This is total size on the disk. Compressed download size was cca 50% of the old image, so I expect similar reduction for the new image.

[davidbilik]

👏