Is there any way to write PID file to /var/run or so with knot-resolver?
I can't find any info in documentation about it.

We don't use PID files ourselves.  I see those as a (poor man's) tool of the init system running the service, so it's the OS init's jobs to use them or not - and I think some knot-resolver services do use them, but not our systemd-based packages.

Also, control sockets get named by PID sometimes.  But again, in our systemd-based packages we use ID of the instance instead of PID, because it just seems easier to use.  https://knot-resolver.readthedocs.io/en/stable/daemon-scripting.html#control-sockets

As configuration file is just a lua scripts, there certainly is a way to write PID into a file on startup, in a few lines of code.