Installing only packages from certain sources to optimize docker builds

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

Feature Request

I'm using poetry for a very large python project which include a lot of internal and external packages.
We build a lot of containers and in the build stage we install the packages using poetry install.
Every time we change our code of internal packages we have to rebuild the container from 0 which takes a lot of time.
We would like to optimize it by copying only the pyproject.toml and poetry.lock files and install the external python packages only for the first stage to have it cache for the next builds.

Similar to how multistage builds work are recommended in npm projects, see example of the multistage builds:
https://cloudnweb.dev/2019/10/crafting-multi-stage-builds-with-docker-in-node-js/

In order to do that we need the ability to specify to install only the external packages (pypi, git etc..) from the lock file without require the internal packages code.

Start with the poetry install workflow and how pyproject.toml and poetry.lock are used during container builds. Determine how an installation can use only external packages from the lock file without requiring internal package code; done means a documented or testable installation path supports this caching workflow.