Docker refactoring

[Dim0N22]

This PR is related to PR#2 in ElementumOrg/cross-compiler.
It introduces a couple of improvements:

• Unified image tags (elementumorg/cross-compiler:base instead of cross-compiler:base)
• Dockerfile uses multistage builds to improve performance and minimize resulting image
• Library versions can be changed independently without affecting others. For example changing boost version won't trigger rebuid for swig.
• If you use Docker 18.09+, you can set DOCKER_BUILDKIT=1 to build multiple indepdendent stages in parallel. This further improves build speed

I've tested it and successfully built libtorrent for android-arm.
This is a draft PR but I want to work on all other images as well.
Let me know what you think.