ASP.NET Core + Docker + Sql Server

Hi Devs,

I´m trying to implement microservices using  this link https://docs.microsoft.com/en-us/dotnet/standard/microservices-architecture/multi-container-microservice-net-applications/data-driven-crud-microservice but I get an error in docker compose file (i´m new in docker). The error says: Service "" has neither an image nor a build context specified. At least one must be provided. I try to google the error but have no success.

Code of docker-compose.yml: 

version: '3'

services:
  dockerExample:
    image: rntestrategia360objectivosestrategicos
    build:
      context: .
      dockerfile: DockerExample/Dockerfile
  DockerExaple.api:
    environment:
      - ConnectionString=Server=sql.data;Database=db;User Id=id;Password=password
      # Additional environment variables for this service
    ports:
      - "5101:80"

Best Regards