Change Dockerfile to run Chproxy under non-root user

[dmeremyanin]

Description

Changes:

1. Security improvement: the application now runs as a non-root user inside the container, following best practices for containerized environments. This helps reduce the potential impact of security vulnerabilities.
2. Image size optimization: switched the base image to debian:12-slim, reducing the overall image size from 291MB to 219MB.

For more information, refer to the following resources:

• Sysdig's Dockerfile Best Practices
• Kubernetes Security Checklist

Fixes https://github.com/ContentSquare/chproxy/issues/515

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [x] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

Checklist

• [x] Linter passes correctly
• [ ] Add tests which fail without the change (if possible)
• [ ] All tests passing
• [ ] Extended the README / documentation, if necessary

Does this introduce a breaking change?

• [ ] Yes
• [x] No