How to set dynamic values with Kubernetes yaml file

For example, a deployment yaml file:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: guestbook
spec:
  replicas: 2
  template:
    metadata:
      labels:
        app: guestbook
      spec:
        container:
          - name: guestbook
            image: {{Here want to read value from config file outside}}

There is a ConfigMap feature with Kubernetes, but that's also write the key/value to the yaml file. Is there a way to set the key to environment variables?