feat: Implement nix-based Dockerfile for CI

[mathbunnyru]

High Level Overview of Change

I implement nixos-packages build Docker images.

This allows us to have the following:

• Modern compilers (gcc / clang)
• unified declaration of tools/packages required for development/building/testing of rippled
• No Docker Images duplication depending on the OS and/or versions
• And the Dockerfile will be stored here, since it'll be less frequently updated and easier to support

All that being said, we use libc of the oldest distro we support.
ubuntu:20.04 had libc 2.31, so we have to:

• build this libc version
• build gcc with this libc
• and libstdc++ as well

This allows us to build binaries compatible with old OS, but have all the features of the modern compilers.

Current limitations (not going to be addressed in this PR, but will be resolved in the future):

• only linux/amd64
• only gcc-15 (clang will be added later)
• not yet tested if rippled is capable of being built in this image, but tools are verified to be working (i.e. we run cmake --version in the image)

The base builder image ends up being the same (nixos), so it gets rebuilt every time.
But let's not worry about it too much - it won't be updated very often, so it shouldn't be too much of a problem.
And it's all in parallel, so we should be good too.

Context of Change

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[kuznetsss]

👍