[dockerfile] Copying local flakes in dockerfile

[mohsenari]

Summary

This PR addresses the issue in generated dockerfiles by devbox generate dockerfile and devbox generate devcontainer in which packages mentioned as local flakes are not copied over in dockerfile and user has to manually do that.
Addresses: #1051

How was it tested?

• compile
• add a local flake (example: https://github.com/NixOS/templates/blob/master/go-hello/)
• add local flake to devbox
• ./devbox generate dockerfile
• check the contents of generated dockerfile and confirm there is a Step 6 comment and COPY commands for each local flake.

[Lagoja]

This Dockerfile doesn't copy any of my code, is that expected?

[mohsenari]

This Dockerfile doesn't copy any of my code, is that expected?

@Lagoja are you adding local flake to devbox.json? for every package that has this format in devbox.json: path:./path/to/flake#name it should add a line in Dockerfile like this: COPY ./path/to/flake ./path/to/flake