Add Docker build and push CI/CD pipeline

[r33drichards]

Summary

This PR adds automated Docker image building and pushing to the project's CI/CD pipeline using GitHub Actions and Nix.

Key Changes

• New GitHub Actions Workflow (.github/workflows/docker-build-push.yml)

  • Triggers on pushes and PRs to master and main branches
  • Builds Docker image using Nix flakes
  • Pushes image to Docker Hub with commit SHA tag on all builds
  • Pushes image with latest tag only on main/master branch pushes
  • Uses skopeo for secure Docker image pushing with credentials

• Updated Nix Flake Configuration (flake.nix)

  • Extracted mcp-exec package into a variable for reusability
  • Added new packages.docker output using dockerTools.buildLayeredImage
  • Docker image includes essential runtime dependencies:
    • bashInteractive - shell support
    • coreutils - standard utilities
    • inetutils - networking tools
  • Configured container with:
    • Entry point: mcp-exec binary
    • Exposed port: 8080/tcp
    • Proper PATH environment variable

Implementation Details

• Uses Nix's dockerTools.buildLayeredImage for efficient, reproducible Docker builds
• Leverages GitHub secrets for Docker Hub credentials (DOCKER_USERNAME, DOCKER_PASSWORD)
• Supports multi-architecture builds (x86_64-linux, aarch64-linux) via existing flake configuration
• Commit SHA tagging enables easy tracking of image versions to specific commits