feat: Add Docker workflow for building, pushing & signing images

[dotWee]

This pull request includes significant updates to automate the Docker image building and pushing process using GitHub Actions, as well as updating the Docker commands in the README.md file.

Automation of Docker image building and pushing:

• .github/workflows/docker.yml: Added a GitHub Actions workflow to build and push Docker images on a schedule, push to specific branches and tags, and handle pull requests. The workflow includes steps for setting up QEMU, installing cosign, setting up Docker Buildx, logging into the registry, extracting Docker metadata, building and pushing the Docker image, and signing the Docker image digest.

Documentation updates:

• README.md: Updated the Docker commands to use the GitHub Container Registry (ghcr.io) instead of a local Docker image. This includes commands for building, pulling, and running the Docker image.

[quentinsf]

Thanks, Lukas - this is all great... but, even though I'm a big fan of Docker and use it all the time, I can't help wondering if it might be overkill for running a single python script which doesn't have any dependencies and can be pip installed... :-)

Still, I'll give it a try soon...