Configure mining credentials and build CGMiner 3.7.2 with GPU support

[drqsatoshi]

This pull request introduces several improvements and modernization efforts for the build process, deployment, and codebase of the project. The most significant changes include adding CI workflows for automated builds and tests, updating Dockerfiles for easier deployment and compatibility, improving Python 3 support, and updating documentation and configuration defaults to use braiins.com instead of slushpool.com. Below are the most important changes grouped by theme:

Build and CI Automation

• Added .github/workflows/build-cgminer.yml to automate building the cgminer 3.7.2 binary with AMD GPU and scrypt support, including artifact upload for easy distribution.
• Added .github/workflows/c-cpp.yml for automated C/C++ CI, running configure, make, and checks on every push and pull request.

Docker and Deployment Improvements

• Replaced the old Dockerfile with a Python 3-based image, updated dependency installation, switched entrypoint to Python 3, and simplified repository handling for better compatibility and maintainability.
• Added Dockerfile.cgminer for multi-stage building of cgminer 3.7.2, producing a minimal runtime image with only necessary dependencies and the cgminer binary.

Python 3 Compatibility and Code Cleanliness

• Updated imports in mining_libs/client_service.py and mining_libs/jobs.py to use relative imports, improving compatibility with Python 3 module structure. [1] [2] [3]
• Modified mining_libs/getwork_listener.py and mining_libs/jobs.py to handle bytes vs. string conversions, ensuring proper encoding/decoding for Python 3 and more robust request processing. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

Documentation and Configuration Updates

• Updated README.md instructions and examples to use braiins.com as the default pool host, added detailed cgminer 3.7.2 build instructions (manual, CI, and Docker), and clarified scrypt mining setup for AMD GPUs. [1] [2] [3]
• Changed default pool host in debian/stratum-mining-proxy.default from slushpool.com to braiins.com.

These changes collectively modernize the project, streamline deployment and build processes, improve cross-platform compatibility, and update documentation for current recommended practices.