ALso install python before using pip in the documentation job

.github/workflows/docker-publish.yml

@@ -37,6 +37,11 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
+    - name: Install python version
+      uses: gabrielfalcao/pyenv-action@v7
+      with:
+        default: "3.6.8"
+        command: "pip install -U pip"
     - name: dependencies
       run: |
         sudo apt-get update -qq