[generate] Added message output for generate dockerfile and devcontainer commands

[mohsenari]

Summary

Added explanation to the docs and a message output to devbox gen dockerfile and devbox gen devcontainer commands that the generated dockefile doesn't copy the whole project files. It only copies devbox.json and devbox.lock. The user needs to modify the dockerfile to include copying their project specific files and directories.

How was it tested?

• devbox run build
• devbox gen
• devbox gen --quiet dockerfile/devcontainer -f