Trying to fix HTTPS connections in the ci container

There must have been some change to the network that the CI is running on and it caused HTTPS connection to github to hang forever. Lowering the MTU on the docker network bridge fixed it for some of the jobs. (I added this to the /etc/docker/daemon.json on our CI server.)

Now only the build_test_env job fails, probably because something on that job overrides the MTU setting. docker-compose apparently doesn't inherit the daemon settings. (https://mlohr.com/docker-mtu/) Could that have to do with it?