fix: tests with Dockerfile (#2382)

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

.github/CONTRIBUTING.md

@@ -123,6 +123,12 @@ To run unit tests:
 make test
 ```
 
+To run unit tests in Docker:
+
+```bash
+make docker_tests
+```
+
 If you add new logic, please add a unit test.
 
 Integration tests cover logic that requires making calls to outside APIs (often integration with other services).