Update the Dockerfile to use the `$POETRY_HOME` argument to set the
Poetry home directory instead of adding Poetry to the PATH environment
variable.
Add instructions to the `CONTRIBUTING.md` file on how to run tests with
Docker.
Closes https://github.com/hwchase17/langchain/issues/2324
When following the Quick Start instructions in the contributing docs, I
was getting a "WheelFileValidationError" on installation of debugpy
which was blocking the installation of a number of other deps. Google
turned up this [GitHub
issue](https://github.com/microsoft/debugpy/issues/1246) indicating a
regression in Poetry 1.4.1 and workarounds.
This PR updates the contrib docs noting the issue and the workarounds.
