Only install dependencies

[alex007sirois]

I recently changed my Dockerfile to use multi-stage builds.
I thought PDM would be perfect because of it's use of a __pypackage__ directory.
When building a docker image, you want the layers in order from least to most susceptible to change.
In that way, we want to install dependencies before the code as to not reinstall them every time the code change.
However, it is not possible right now with PDM.

Example:

FROM python AS builder

WORKDIR /

RUN pip install pdm
COPY pdm.lock
pdm install --only-dependencies  # <-- this flag does not exist in PDM 1.5.2

FROM python

ENV PYTHONPATH=src:packages
COPY --from=builder /__pypackage__/3.9/lib packages
COPY src src

ENTRYPOINT ["python", "src/main.py"]

Describe the solution you'd like

Poetry has a --no-root option when installing that has this behavior.

By default, the above command will also install the current project. To install only the
  dependencies and not including the current project, run the command with the
  --no-root option like below:

   poetry install --no-root

I think adding a similar flag to pdm install would be best.
Although, I find that name confusing and would prefer something like --only-dependencies, but I don't really mind, as long as such an option exists.

[pawamoy]

I see @frostming is working on this issue as well as #443 in #456 ❤️