US-3: Docker multi-stage build and Docker Compose deployment

[arampp]

Epic

Part of #1 (EPIC-1 — Project Scaffold & Infrastructure)

User Story

As the owner self-hosting Procastimarks,
I want to start the application with a single docker compose up command,
so that deployment and upgrades require no manual steps.

Acceptance Criteria

• docker build . succeeds and produces an image < 200 MB
• Multi-stage Dockerfile: stage 1 compiles the Rust binary; stage 2 is a minimal
  runtime image containing only the binary and static assets
• docker compose up starts the container and the /health endpoint responds
• SQLite database file is stored on a named Docker volume (survives docker compose down without -v)
• README (or compose file comment) explicitly warns that docker compose down -v
  destroys all data (C-8)
• API_KEY environment variable is passed via docker-compose.yml (or .env file)

INVEST

Criterion	Notes
Independent	Can be started in parallel with US-8 after US-7
Negotiable	Base image choice is open
Valuable	Delivery mechanism for the self-hosted product
Estimable	~half a day
Small	Infra only; no feature logic
Testable	docker compose up + health check is concrete

Technical Notes (arc42 §7)

• cross or cargo-leptos build inside the Docker build stage
• WASM assets copied into the runtime image
• Named volume mounted at /data; DATABASE_URL=/data/procastimarks.db
• ATAM condition C-8: data-loss warning must be present in docs

Dependencies

• Depends on: US-1: Cargo workspace and Leptos project scaffold #7 (US-1)

MoSCoW: Must Have