Update pipenv Dockerfile to fix build error.

They removed pipenv lock -r, so we need to update it

pypa/pipenv#5253

cppseminar/vm-test-server/Dockerfile

@@ -7,7 +7,7 @@ WORKDIR /app/src
 COPY . .
 
 RUN pip install pipenv \
-    && pipenv lock --keep-outdated --requirements > requirements.txt \
+    && pipenv requirements > requirements.txt \
     && pip install -r requirements.txt
 
 ENTRYPOINT ["python", "./main.py"]