Add --only-root option for install command.

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

It would be nice if the --only-root option for install command will be introduced. We have a use-case with the multistage docker build. At first, we install dependencies with --no-root option because dependencies are not changing so often. During the second stage, we would like to install only the application. Unfortunately, there are some git dependencies so the second stage requires git installed even no dependencies are finally installed.

[brettdh]

Also interested in this feature. I have a similar multistage build, with the added constraint that the second stage has no network connectivity, so as to isolate the parts of the build that depend on external data. We could remove --no-root from the first-stage installation, but this would require moving build steps to the first stage as well, which defeats the purpose of them taking place in a second, isolated environment. Current workaround is to run poetry install during the second stage as well - which works, as long as the venv has not changed between the stages - but it makes me uneasy that I can’t explicitly say “only install the root package”.

[sh-at-cs]

This seems to have been implemented recently in #5783.