[ENHANCEMENT] Use non-privileged 'chainlink' user by default in Dockerfile

[RinZ27]

Description

The official Chainlink Dockerfile currently defaults to the root user (ARG CHAINLINK_USER=root), even though the image contains logic to create a non-privileged chainlink user. Running containers as root by default is a security risk and violates the principle of least privilege.

Proposed Improvement

Switch the default CHAINLINK_USER to chainlink. Users who explicitly need root can still override the argument, but the "secure by default" path should be a non-privileged user.