title: `docker-compose up` context not specified
body: <p>When I run my docker-compose.yml file using <code>docker-compose up</code>, it outputs:</p>
  
  <pre><code>ERROR: The Compose file is invalid because:
  Service webserver has neither an image nor a build context specified. At least one must be provided.
  </code></pre>
  
  <p>Before running <code>docker-compose up</code> I ran <code>docker build .</code> using a Dockerfile that was successful. So far i've only tried quitting, restarting Docker, and using <code>docker system prune -a</code> to remove docker images, containers, volumes, and networks.</p>
  
  <p>Here's the Dockerfile:</p>
  
  <pre><code>WORKDIR /usr/local/airflow/
  COPY requirements.txt ./
  RUN pip install --upgrade pip &amp;&amp; \
      pip install --no-cache-dir -r requirements.txt
  COPY . .
  </code></pre>
  
  <p>Here's the docker-compose.yml file:</p>
  
  <pre><code>version: '3.7'
  services:
      postgres:
          image: postgres:9.6
          environment:
              - POSTGRES_USER=airflow
              - POSTGRES_PASSWORD=airflow
              - POSTGRES_DB=airflow
          logging:
              options:
                  max-size: 10m
                  max-file: "3"
  
      webserver:
          # image: puckel/docker-airflow:1.10.9
          image: puckel/docker-airflow:latest
          restart: always
          depends_on:
              - postgres
          environment:
              - LOAD_EX=n
              - EXECUTOR=Local
          logging:
              options:
                  max-size: 10m
                  max-file: "3"
          volumes:
              - ./dags:/usr/local/airflow/dags
              # - ./plugins:/usr/local/airflow/plugins
              - ./plugins/:/usr/local/airflow/plugins
              - ./requirements.txt:/requirements.txt
          ports:
              - "8080:8080"
          command: webserver
          healthcheck:
              test: ["CMD-SHELL", "[ -f /usr/local/airflow/airflow-webserver.pid ]"]
              interval: 30s
              timeout: 30s
              retries: 3
  
          build:
              args:
                  PYTHON_DEPS: "boto3==1.12.41, notebook==6.0.3, numpy==1.18.1, pandas==0.25.3, psycopg2==2.8.4"
                  AIRFLOW_DEPS: "aws, postgres"
  </code></pre>
  
  <p>Here's the requirement.txt file:</p>
  
  <pre><code>boto3==1.12.41
  notebook==6.0.3
  numpy==1.18.1
  pandas==0.25.3
  psycopg2==2.8.4
  </code></pre>