Update docker.opts and inform users of create_docker.py

What this MR does / why does DuMux need it:

docker.opts contains cmake options more suitable for docker containers. These haven't been updated in a while. To use them when generating a docker container using the DuMux scripts one must pass the opts file to make_installscript.py before using this in create_docker.py. This should be made more transparent.

Notes for the reviewer

Not sure how best to inform/remind users of the docker script. What other content of docker.opts should be updated?

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