and I run agent from local user and I would expect that settings to have effect -v /home/localuser/.ssh:/home/testuser/.ssh

It does not map it directly, it creates a temp copy in the host /tmp folder of the entire ".ssh" folder, than maps this folder inside the container:
https://github.com/allegroai/clearml-agent/blob/a5a797ec5e5e3e90b115213c0411a516cab60e83/clearml_agent/commands/worker.py#L3422
Notice that the "docker_internal_mounts" section is nested inside the "agent" section in the clearml.conf file.
Can you share the top Task log of a task your agent is running? (the first few lines contain the exact docker run command the agent is using)