improve build scheme

[korniltsev]

• [x] Build and publish a docker image based on manylinux with a preinstalled cargo, libunwind and all dependencies (#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 (#83)
• [x] https://github.com/pyroscope-io/pyroscope-rs/issues/74
• [ ] maybe crosscompile to macos from linux

[korniltsev]

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