title: ECS invalid reference format error
body: <p>I try to deploy and run a simple Dockerized Ruby backend using Amazon ECS by running a Task (defined by Task Definition) but it failed to run my command and returned this error:</p>
  
  <blockquote>
    <p>CannotPullContainerError: API error (400): invalid reference format</p>
  </blockquote>
  
  <p>I've tried to run a small Entrypoint command of <code>echo "Hello"</code> but it's returning the same error regardless it's exec form or shell form. I tried to run this command in my local terminal it's running well:</p>
  
  <pre><code>$ docker-compose run job echo 'HELLO_WORLD'
  HELLO_WORLD
  </code></pre>
  
  <p>This is the error message as shown in the Details section of my cluster task:</p>
  
  <pre><code>Status reason   CannotPullContainerError: API error (400): invalid reference format
    Entry point   ["echo","HELLO"]
  </code></pre>
  
  <p>this is my Task Definition JSON (hiding certain information for security reason):</p>
  
  <pre><code>{
      "executionRoleArn": null,
      "containerDefinitions": [
          {
              "dnsSearchDomains": null,
              "logConfiguration": null,
              "entryPoint": [
                  "echo",
                  "HELLO"
              ],
              "portMappings": [],
              "command": [],
              "linuxParameters": null,
              "cpu": 0,
              "environment": [
                  {
                      "name": "RAILS_ENV",
                      "value": "production"
                  }
              ],
              "ulimits": null,
              "dnsServers": null,
              "mountPoints": [],
              "workingDirectory": null,
              "dockerSecurityOptions": null,
              "memory": 7000,
              "memoryReservation": null,
              "volumesFrom": [],
              "image": "&lt;ecs_image_arn&gt;",
              "disableNetworking": false,
              "healthCheck": null,
              "essential": true,
              "links": null,
              "hostname": null,
              "extraHosts": null,
              "user": null,
              "readonlyRootFilesystem": null,
              "dockerLabels": null,
              "privileged": null,
              "name": "&lt;name&gt;"
          }
      ],
      "memory": null,
      "taskRoleArn": "arn:aws:iam::&lt;id&gt;:role/&lt;name&gt;",
      "family": "&lt;test-cluster&gt;",
      "requiresCompatibilities": [
          "EC2"
      ],
      "networkMode": null,
      "cpu": null,
      "volumes": [],
      "placementConstraints": []
  }
  </code></pre>
  
  <p>Changing entryPoint to use command doesn't work as well.</p>
  
  <pre><code>Status reason   CannotPullContainerError: API error (400): invalid reference format
      Command ["echo","HELLO"]
  </code></pre>
  
  <p>Am I missing out any configuration to make a simple <code>echo</code> command work?</p>