Improve Dockerfile and add Docker pipeline

[ogarcia]

This pull request introduces a new GitHub Actions workflow for building and publishing the TierDrop Docker container and refactors the docker/Dockerfile to improve the build process, dependency management, and container layering. The changes streamline the Docker image build, separate concerns for building TierDrop and installing ZeroTier, and automate image publishing to GitHub Container Registry.

CI/CD Automation:

• Added .github/workflows/docker-publish.yml to automatically build and publish the TierDrop Docker image on every push or tag, including steps for multi-architecture builds, Docker metadata extraction, and pushing to GitHub Container Registry.

Dockerfile Refactoring and Improvements:

• Refactored docker/Dockerfile to use multi-stage builds: one stage for building TierDrop, another for installing ZeroTier, and a final stage to assemble the runtime image, resulting in a smaller and more maintainable container.
• Explicitly pulls base images from docker.io for clarity and reproducibility.
• Installs only minimal dependencies in the final image and ensures proper permissions and directory setup for ZeroTier.
• Copies only necessary binaries from the builder and ZeroTier stages, reducing image size and attack surface.

Of course, feel free to make any changes you see fit.