Docker-compose roles reusing same image

docker-compose.yml

db:
  image: postgres:9.4.1
  ports:
    - "5432:5432"
redis:
  image: redis
worker:
  build: .
  command: bundle exec sidekiq
  environment:
    RAILS_ENV: development
    RESUMIS_CANONICAL_HOST: resumis-docker.dev
    RESUMIS_TENANCY_MODE: single
    DATABASE_URL: postgres://postgres@db/resumis_development
    REDIS_URL: redis://redis:6379/1
  links:
    - db
    - redis
  volumes:
    - .:/resumis
web:
  build: .
  command: bin/rails server --port 3000 --binding 0.0.0.0
  environment:
    RAILS_ENV: development
    RESUMIS_CANONICAL_HOST: resumis-docker.dev
    RESUMIS_TENANCY_MODE: single
    DATABASE_URL: postgres://postgres@db/resumis_development
    REDIS_URL: redis://redis:6379/1
  ports:
    - "3000:3000"
  links:
    - db
    - redis
  volumes:
    - .:/resumis