gleam: add Dockerfile and docker-compose entry

[HeyItWorked]

Task

Create gleam-bookshelf/Dockerfile and add the gleam-bookshelf service to the root docker-compose.yml.

Dockerfile

• Base image: ghcr.io/gleam-lang/gleam:v1.9-erlang-alpine (or latest)
• Build: gleam export erlang-shipment
• Run: build/erlang-shipment/entrypoint.sh run
• Expose port 8082

docker-compose.yml addition

```yaml
gleam-bookshelf:
build: ./gleam-bookshelf
ports:
- "8082:8082"
environment:
DATABASE_URL: postgres://shelf:shelf@db:5432/bookshelf?sslmode=disable
depends_on:
db:
condition: service_healthy
```

Acceptance criteria

• docker compose build gleam-bookshelf succeeds
• docker compose up gleam-bookshelf starts and connects to shared Postgres
• API responds on http://localhost:8082/books