Add workflow for automated Docker builds on project changes

[marwan37]

This PR adds a GitHub Actions workflow that automatically builds and pushes Docker images to our zenmldocker account on DockerHub when changes are made to project directories.

How the workflow works:

• Builds Docker images only for projects that have changes
• Uses path-ignore pattern to exclude any root files and dirs that aren't project directories
• Supports manual triggering for specific projects
• Automatically generates Dockerfile.sandbox for projects that don't have one using the generate_sandbox_dockerfile.py script
• Commits generated Dockerfiles back to the repository after successful builds (may need to set GITHUB_TOKEN in our repo settings if it's not already there)
• Pushes images to DockerHub with appropriate tags

[marwan37]

Closing this. Adding it to #196