Improve docker-localnet workflow

[basfroman]

Context:

Improve Docker build flow for Subtensor Localnet

• Created install_build_env.sh with a clear warning about its usage in .github/workflows/docker-localnet.yml and Dockerfile-localnet.
• Parallelized the build of fast-runtime and non-fast-runtime binaries using a matrix strategy.
• Ensured artifact layout matches expected Dockerfile-localnet paths.
• Added logic to skip build and dependency installation when BUILT_IN_CI is set (when build happened on GH Actions). Otherwise, regular local build is going in the same way.

Result:

• Total build time has been reduced from over 3 hours to about 40 minutes.
• Local Docker build completed successfully.
• Workflow triggered via workflow_dispatch also completed successfully and the Docker image was published without issues.
• SDK tests passed successfully when running locally on macOS with M3 and when running in GitHub Actions in the cloud.