How to set dynamic values with Kubernetes yaml file

For example, a deployment yaml file:

apiVersion: extensions/v1beta1kind: Deploymentmetadata:  name: guestbookspec:  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?