Adjust CI again to try to get around the next mount issue we hit with…

… the adjusted 'bind,ro' option. AFAICT from discussion online docker-compose does not always like relative ipaths in volume sections. Local experiments indicate that this may also be the case here.

.github/workflows/ci.yml

@@ -33,6 +33,9 @@ jobs:
       - name: Build images
         run: |
           make init
+          echo "" >> .env
+          echo "# NOTE: try absolute path to current dir to get docker ro-mount to work" >> .env
+          echo "DOCKER_MIGRID_ROOT=$PWD" >> .env
           make dockerbuild
       - name: Start services
         run: |