Set the CLI build version from OBSCURA_VERSION, GitHub tag refs, or Cargo.toml fallback. Use that same build version for both --version and the startup banner, and pass the Docker tag version into Docker builds.
Adds a multi-stage Dockerfile (rust:1-slim-bookworm builder → distroless/cc
runtime) plus a --host flag for obscura serve that defaults to 127.0.0.1
for safety but can be set to 0.0.0.0 so the container's port is reachable
via docker run -p mapping.
Dockerfile cherry-picked from #84. The SPA chunk loading commit from that
PR was left out — verified it didn't actually fix the Next.js webpack
errors on kiwi.com (#86).