add Dockerfile technical reference

[Vatson112]

Add docs about Dockerfile structure and tools.

Reference: rustic-rs/rustic#969

[simonsan]

Could you run dprint fmt on your PR, please so we can merge it? :)

[Vatson112]

@simonsan sure, done.

[simonsan]

I converted it to a draft, because I want to go over it as well, the next days/week.

Especially, I think it's worth it to explain better/fix what's missing:

• maybe even move to an own markdown file in the file system
• why we use it
• what we use it for in the rustic ecosystem
• where the Dockerfile is located
• what the requirements are (e.g. podman)
• some useful commands to get started with development

@Vatson112 Feel free to add any of these yourself if you want. I will chime in along the lines. 🚀