build.yml: use cache for Docker build envs

[plowsof]

after 1st run, the cache will be used. the docker* workflows will build in under 15 mins , example @ https://github.com/plowsof/monero-gui/actions/runs/11232245513

threads bumped to 4

this is not a fancy docker layer caching method, which isnt a concern as if any of the Dockerfile.* are modified then the build env will be built from scratch.

every github repo has 10GB of cache to play with

docker-windows-static Cache Size: ~706 MB (740221653 B)
docker-linux-static Cache Size: ~584 MB (612024901 B)
docker-android-static Cache Size: ~2231 MB (2339233117 B)

1st run: https://github.com/monero-project/monero-gui/actions/runs/11232547452
2nd: https://github.com/monero-project/monero-gui/actions/runs/11233958792