Optimize image

Currently image has about 1,42 GB so probably there are a lot of files unnecesary for last step - firmware build.

This could be solved by dividing Dockerfile to multiple stages, so unnecesary artifacts will not be included in final image. Also last stage possibly will not needed to be based on huge distro.

Now it is needed to pinpoint files and directories what are needed by last stages (that intended to be run by image user - cd micropython/ports/esp8266 && make or cd micropython/mpy-cross && make).

Helpful reports

• du-all.txt - all directories in image with size sum
• du-over-100MB.txt - the same but filtered to directories bigger than 100 MB