The elegant method of activating virtualenv in Python container

Dockerfile

FROM ubuntu:18.04
RUN apt-get update && apt-get install \
  -y --no-install-recommends python3 python3-virtualenv

ENV VIRTUAL_ENV=/opt/venv
RUN python3 -m virtualenv --python=/usr/bin/python3 $VIRTUAL_ENV
ENV PATH="$VIRTUAL_ENV/bin:$PATH"

# Install dependencies:
COPY requirements.txt .
RUN pip install -r requirements.txt

# Run the application:
COPY myapp.py .
CMD ["python", "myapp.py"]