Prebuild images on ghcr.io

[AlvaroBrey]

For faster local usage, we may want to prebuild the images on GH actions. ghcr.io should be free for public repos.

[AlvaroBrey]

• master image should be automatically rebuilt nightly
• latest stable should be rebuilt weekly

[AlvaroBrey]

Also make sure that images are always pulled in start.sh to ensure up-to-dateness