fix(docker): Run ubuntu container as non-root user for security

[vibemasshq-dev]

Hey team! Exploring the repo and noticed a quick security tweak for the Docker environment.

1. Security Fix: Docker Container Runs as Root

Currently, the etc/docker/ubuntu.Dockerfile does not specify a user, meaning processes run as root by default. This increases the container's attack surface.
Fix: Added a non-root appuser and switched the container context to run under it safely.

2. Architecture Heads-Up (No Action Required)

As a side note, I was doing some static analysis on the codebase and noticed a potential connection to Issue #2221 (the git grep panic).

The analysis flagged an unhandled unwrap() call in src/handlers/grep.rs at line 197 (and 338) on grep_line.submatches. If the grep output doesn't match exactly what's expected, this will panic the main thread. This perfectly aligns with the stack trace reported in that issue!

Also, files like line_numbers.rs and diff_header.rs are getting quite large and might be prime candidates for splitting into smaller modules soon.

Awesome tool, I use it every day! 🚀

[vibemasshq-dev]

Any review on this? Thank you