consider building and pushing interop runner image in CI

[marten-seemann]

I noticed that the lsquic interop runner image is now almost 1 year (!) old: https://hub.docker.com/r/litespeedtech/lsquic-qir/tags

May I suggest to push a new interop runner on every push to master, or on a daily (?) cron job? GitHub Actions has some predefined workflows that should make building and pushing Docker images pretty straightforward. For inspiration, here is quic-go's workflow, and Quinn has one as well.

[litespeedtech]

We will look into that. Thanks!