chore: add Docker support

[mirusu400]

BlocksDS officially supports a Docker image, so I added a Dockerfile to build the project easily.
I have tested this on an Apple Silicon Mac.

docker build --platform linux/amd64 -t pico-loader-builder . 
docker run --rm --platform linux/amd64 -v "$(pwd):/workdir" pico-loader-builder
docker run --rm --platform linux/amd64 -v "$(pwd):/workdir" -e TARGET_PLATFORM=R4 pico-loader-builder

...

sha256sum picoLoader9_R4.bin
aafb234966cf29c7ebdc111699fb683a9157067a45950e56f89cc6dfb257e1c8  picoLoader9_R4.bin