Kubernetes configuration file: add environment variable for mini-app

[ptitzler]

Users can disable the mini-app endpoint by setting environment variable DISABLE_WEB_APP to true.
We should add this variable to the object-detector.yaml configuration file as follows:

spec:
 restartPolicy: Always
 containers:
 - env:
   - name: DISABLE_WEB_APP
     value: "false"
   name: object-detector
   image: codait/max-object-detector

Doing so will not change the default behavior (which is to expose the mini-app) but make it easy for users to do so, by simply changing the value from false to true.