improve build scheme

[korniltsev]

• Build and publish a docker image based on manylinux with a preinstalled cargo, libunwind and all dependencies (fix: use prebuilt docker images based on manylinux with preinstalled cargo & rust #84)
• Build the ruby gem and python wheel inside docker, not a shell script (using buildkit out - see below)
• Make it so that anyone can build a gem or wheel with a single command and then gh actions uses the same command (currently it is multiple commands in gh actions, build this and that, then copy files, then build the package)
• Make thread_id a part of a rbspy library so that we have a single so instead of two
• improve gh actions build time
• make our gem being able to build from source when platform is not specified (installing pyroscope 0.5.1 in bundler #83)
• when I push to a branch it starts gh actions twice #74
• maybe crosscompile to macos from linux

[korniltsev]

use scratch + buildkit output
https://reece.tech/posts/extracting-files-multi-stage-docker/