CI: make sure that demos work

[pepyakin]

Demos are not in the CI and it's quite cumbersome to run them. Those should be checked in the CI.

It's a bit heavy though, so maybe we should run them only on master commits or maybe as part of merge queue as opposed to for every commit on PR. I think I am OK to break master occasionally for the time being if that improves CI times. When the project is released we should be more strict about that though.

Perhaps, that would require us to write docker-compose files (or maybe something similar). Those would allow us a one-click way to launch the demos, which is useful to do to check stuff locally. Those files can also double as configuration examples should the user want to build their own test environments.