Adds support for easy building Nextor with Docker

[gilbertfrancois]

Adds support for easy building Nextor with Docker.

Dear Konamiman,

I wanted to build the latest Nextor on my machine. However, I'm running on a mac with arm processor and saw that Linux is the preferred OS for building this project. That's why I created this docker image (for arm64 and x86_64). Maybe it is useful for other users too, running e.g. Windows, macOS or even Linux, without having the required build tools installed on their system. That is why I propose this pull request.

The only step needed to build Nextor is running the script build_nextor.sh. The ROMs will be available in the dist directory after compilation.

I hope you will consider merging it in your great project.

PS: It was nice to meet you in Amsterdam in december :)

[Konamiman]

Hi @gilbertfrancois, thanks for your contribution and sorry for the long delay in looking into it. Your pull request looks interesting, however I have a couple of comments/change requests:

1. First it's about the organization of the files. Instead of the build_nextor.sh script living directly in the source of the repository, I think it could go into a buildtools/docker directory. Similarly, build_builder.sh and what's inside your docker directory could go in buildtools/docker/internal. This way everything gets neatly organized and it's clear that build_nextor.sh is the one to run.
2. Instead of generating the ROMs in a dist directory, could them go into bin\kernels, as the current makefile does?
3. I see that build_nextor.sh builds NEXTOR.SYS and the tools too, and apparently they are copied to bin/tools, but actually they aren't.
4. If I run the script twice I would expect to nothing happen the second time, since no source files have changed; but actually a full rebuild is triggered again. Could this be fixed?

Let me know what you think, and thank you!