Use arg/env variable for venv creation instead of hardcoded location

Explicitly call the venv version of pip3 to install the wheel, so it is obvious the built prez module is being installed in the venv site_packages.

Dockerfile

@@ -28,8 +28,8 @@ WORKDIR /app
 COPY . .
 
 RUN poetry build
-RUN python -m venv --system-site-packages /opt/venv
-RUN pip install --no-cache-dir dist/*.whl
+RUN python3 -m venv --system-site-packages ${VIRTUAL_ENV}
+RUN ${VIRTUAL_ENV}/bin/pip3 install --no-cache-dir dist/*.whl
 
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 # Final