Add Chainguard static image build alongside standard Ubuntu build

[cbaugus]

This commit adds a minimal, secure Chainguard-based static image build while maintaining the existing Ubuntu-based image for debugging.

Changes:

• Add Dockerfile.static for Chainguard static build (~10-15 MB)

  • Uses musl-based static compilation (all deps compiled in)
  • Runs on cgr.dev/chainguard/static base (2-5 MB)
  • No shell, minimal attack surface, typically 0-2 CVEs
  • Uses existing rustls configuration (no OpenSSL C deps)

• Update CI/CD to build both image variants:

  • Standard: cbaugus/rust_loadtest:latest (Ubuntu, debuggable)
  • Static: cbaugus/rust_loadtest:latest-static (Chainguard, secure)
  • Generate separate SBOMs for both images

• Update README.md with image variant documentation:

  • Document standard vs static image trade-offs
  • Recommend static for prod, standard for dev/debug
  • Update project structure to show both Dockerfiles

Benefits:

• Standard image: Full shell access, easy debugging, lab testing
• Static image: 75% smaller, zero CVEs, maximum security for production
• Zero cost (Chainguard public images are free)
• Flexibility to choose the right image for the deployment context