Use ADD properly to copy source files into the container (#24)

Also re-arrange things in case GH ever starts to cache image layers.

Dockerfile

@@ -11,15 +11,14 @@ RUN apt-get -qy update && apt-get -qy install git && \
     rm -rf /var/cache/apt/* /var/lib/apt/lists/*
 
 WORKDIR /app
-ENV PYTHONPATH "/app:${PYTHONPATH}"
+
+RUN python -m pip --disable-pip-version-check --no-cache-dir --quiet install poetry && poetry config virtualenvs.create false
 
 COPY pyproject.toml .
 COPY poetry.lock .
-RUN python -m pip install --quiet poetry
-RUN poetry config virtualenvs.create false
-RUN poetry install --quiet --no-interaction --no-ansi --no-dev
+RUN poetry install --quiet --no-interaction --no-ansi --no-dev --no-root
 
-ADD release_often .
+ADD release_often ./release_often
 
 # During debugging, this entry point will be overridden. For more information, refer to https://aka.ms/vscode-docker-python-debug
 ENTRYPOINT ["python", "-m", "release_often"]