[docker] Multi-stage Dockerfile

[yanissrairi]

Closes #144.

This rewrites the Dockerfile as a two-stage build: a builder that compiles
forefire and the Python bindings, and a runtime stage that just copies the
artifacts in. The toolchain (build-essential, cmake, python3-dev, netcdf-dev
headers) no longer ends up in the runtime image.

Numbers I just remeasured on Fedora 43, Docker 29:

	master	multistage
disk usage on host	1.38 GB	779 MB (-44%)
pull size	360 MB	195 MB (-46%)

tests/runff/ff-run.bash returns EXIT=0 on the multistage image.

I also dropped tools/devops/Dockerfile.base, Dockerfile.multistage,
readme.md and entrypoint.sh since @antonio-leblanc confirmed in #144 that
those were abandoned attempts. Left tools/devops/increment-version.yml
alone, since it looks intentionally paused (moved out of .github/workflows/
in 2c337fa with the message "not needed now"). Happy to handle it in a
separate issue if you'd like.

[antonio-leblanc]

LGTM
nice results thanks