Try to make pip and pipenv use the installed python version

.github/workflows/docker-publish.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04]
-        python-version: [3.6]
+        python-version: [3.6,3.7,3.8]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
@@ -20,10 +20,13 @@ jobs:
     - name: Tests
       env: 
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        MATRIX_PYTHON_VERSION: ${{strategy.matrix.python-version}}
       run: |
+        echo "Selected python version from matrix is: ${MATRIX_PYTHON_VERSION}"
         sudo apt-get update -qq
         sudo apt-get install -qq libpoppler-cpp-dev unpaper tesseract-ocr imagemagick ghostscript optipng
         pip install --upgrade pipenv
+        pyenv global system ${MATRIX_PYTHON_VERSION}
         pipenv install --system --dev
         cd src/
         pipenv run pytest --cov