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 #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)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

• 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
• No

[dmeremyanin]

If the following features are used:

1. Autocert cache
2. File-based caching

Then the corresponding directories must be writable by UID 1000. This change may be considered breaking, as it requires explicit directory permissions when these features are enabled.