stepup-middleware cache dir needs to be writable

[phavekes]

A quick fix is docker compose exec middleware chown -R www-data /var/www/html/var/cache/prod/ but this should be fixed in the component or during the creating of the image

[quartje]

That's already in the image: https://github.com/OpenConext/Stepup-Middleware/blob/develop/docker/Dockerfile.prod

This is caused by the manual cache clear that is run when setting up the environment. I've added it to the README.md