Add egress proxy container with automation

[JAORMX]

Summary

Adds a minimal and secure egress proxy container based on Squid with full automation support.

Changes

Container Setup

• Dockerfile: Minimal Alpine Linux 3.22.0 base with Squid proxy
• Security: Non-root execution, minimal attack surface, portable healthcheck
• Configuration: Runtime configuration support (no baked-in config)
• Process Management: Uses ENTRYPOINT for proper container lifecycle

Automation

• GitHub Actions: Added egress-proxy-image-build-and-publish job
• Release Strategy: Only pushes to registry on tag releases
• Multi-platform: Supports linux/amd64 and linux/arm64
• Security: Includes Cosign image signing
• Registry: Images published to ghcr.io/stacklok/toolhive/egress-proxy

Development Tooling

• Taskfile: Added build-egress-proxy and build-all-images targets
• Local Development: Uses --load flag for immediate Docker daemon availability
• Integration: Builds alongside main toolhive container

Usage

# Local development
task build-egress-proxy

# Build all images
task build-all-images

# Production releases
# Automatic on git tags via GitHub Actions

Testing

• Local container build
• GitHub Actions workflow validation
• Multi-platform build verification
• Image signing verification

Security Considerations

• Minimal Alpine base image reduces attack surface
• Non-root execution prevents privilege escalation
• Runtime configuration prevents config leakage
• Signed images ensure supply chain integrity
• Portable healthcheck works in isolated networks