Extend dagger shell lab

[chrira]

• start shell with dagger instead of dagger shell
• add variable example (container = ... ${container} ...)
• see https://dagger.io/blog/a-shell-for-the-container-age-introducing-dagger-shell
  • take it as inspiration or link it

There are TODO's in the lab file: https://github.com/puzzle/dagger-techlab/blob/main/content/en/docs/02/2.3/_index.md?plain=1#L237