Add Docker container image build configuration

[treblereel]

Context

Part of epic #1 — Build production-ready REST microservice with k8s deployment on top of casehub-engine. Service needs containerization for deployment in Docker and Kubernetes environments.

What

Create Docker container image build configuration:

• Dockerfile using multi-stage build or Quarkus container-image extension
• Production-optimized image (minimal base, non-root user)
• Container runs casehub-flow application with proper entry point

Acceptance Criteria

• Container image builds successfully using quarkus-container-image-docker or Dockerfile
• Image uses distroless/minimal base image (not full JDK) for production
• Image runs as non-root user for security
• JVM mode image (native image can be follow-up enhancement)
• Image includes only runtime dependencies (no build tools)
• Build produces tagged image: casehubio/flow:latest and casehubio/flow:{version}
• Image can be run standalone with docker run for local testing
• Documentation shows how to build and run image locally

Notes

• Use Quarkus JVM mode for first iteration (faster builds, easier debugging)
• Consider using eclipse-temurin:17-jre or similar minimal JRE base
• Image should support environment variable configuration from issue Add production configuration profile with environment variables #9
• Graceful shutdown configured (quarkus.shutdown.timeout)
• File paths: Dockerfile or src/main/docker/Dockerfile.jvm