Rootless + smaller image + cleanup

[chevdor]

This PR looks bigger than it is, due to the removal of leading spaces...

Here are the changes:

• all the doc moves to it own subfolder
• the scripts also gets a cosier home
• container files (like Dockerfile) move to a new container subfolder
• the container image (even though it was not the goal) shrinks from 582MB to 405MB
• the container is now rootless 🎉
• addition of a justfile, it is convenient for those using it, inert for others. It provides also a nice doc and shows how to build the images. See below.

justfile

Users not using just can just ignore.
For others, you need to set your IMAGE=... in the .env file. It will typically look like IMAGE=your_name/lndg.
Alternatively, you may provide your own registry: IMAGE=docker.io/chevdor/lndg.
After that, you may just 😄 run: just build_image and just push_image.

WARNING: Change for Docker users lndg is now in /lndg/ and no longer /app.