Clean up Docker containers

[elopez]

This PR introduces several changes to the Dockerfile and GitHub Actions used to build the echidna docker container to achieve the following:

• Single point of distribution: Docker Hub is dropped, and images are only published on GitHub Container Registry now
• Smaller images: two variants are introduced, debian (~160M) which ships with a shell, apt-get and nvm, and distroless (~100M) which is just echidna and python dependencies. The current docker image is ~680M.
• Images with metadata and automatic tagging: by leveraging docker/metadata-action.
• Caching: building the images now leverages the gha (GitHub Actions) cache type to reduce build times.

The README section on Docker is also updated to account for the changes in this PR.