fix: docker build

[franklinkim]

Description

Restore Debian base image and fix .dockerignore to unbreak Docker builds.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• 🏃‍➡️ Performance
• ✅ Tests
• 🔐 Security
• 🔧 Build/CI

Changes

• Switched Dockerfile FROM back to debian:stable-slim pinned to digest sha256:eb593cf2c358cacef45ca0a424bbc7d30cfa3466265fc2662b9466a0ca6ba1c5
• Removed incorrect Alpine base image that lacked ocproxy package needed by openconnect
• Fixed .dockerignore to exclude bin/ instead of dockprox, preventing accidental deletion of the binary needed for COPY dockprox /usr/bin/

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.