Install git in container used for automatic userguide generation (#254)

* gh action uses image without git: therefore add it, provide the newest moderncv_userguide.pdf manually due to failed action

* update apt sources

.github/workflows/build-userguide-pdf.yml

@@ -45,6 +45,8 @@ jobs:
       - name: Push new user guide to master
         if: ${{ github.ref_name == 'master' }}
         run: |
+          # maybe switch to an image with git already included, for now install it here
+          apt update && apt install git -y
           git config --global user.name '${{ env.UG_AUTHOR }}'
           git config --global user.email '${{ github.actor }}@users.noreply.github.com'
           git add manual/moderncv_userguide.pdf