Variable references are possible using the $(ENV_VAR) format. So, in this example, it would be:

containers:
 - env: 
     - name: DATA_SOURCE_PASS
       valueFrom:
         secretKeyRef:
           name: app-secrets
           key: DB_DOCKER_USER_PASSWORD
     - name: PG_EXPORTER_URL
       value: postgres://postgres:$(DATA_SOURCE_PASS)@localhost:5432/postgres

• This question has already been answered here, which is where I got this answer from.
• Kubernetes documentation on variable references, also linked from article above.