Update Dockerfile

* Update poetry URL
* Use a virtual environment to install dependencies, fix nornir-automation#464
* Use slim-buster instead of slim-stretch

Author: dmfigol

Dockerfile

@@ -1,16 +1,16 @@
 ARG PYTHON
-FROM python:${PYTHON}-slim-stretch
+FROM python:${PYTHON}-slim-buster
 
 WORKDIR /nornir
-ENV PATH="/root/.poetry/bin:$PATH" \
+ENV PATH="/nornir/.venv/bin:/root/.poetry/bin:$PATH" \
     PYTHONDONTWRITEBYTECODE=1 \
     PYTHONUNBUFFERED=1 \
     NORNIR_TESTS=1
 
 RUN apt-get update \
     && apt-get install -yq curl git pandoc \
-    && curl -sSL https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py | python \
-    && poetry config virtualenvs.create false
+    && curl -sSL https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py | python \
+    && poetry config virtualenvs.in-project true
 
 COPY pyproject.toml .
 COPY poetry.lock .