S3 trigger for existing bucket

Serverless Framework
,
1

I am trying to deploy AWS Lambda function which is triggered when image is uploaded to S3 bucket. I tried following

service: image-resize

plugins:
  - serverless-plugin-include-dependencies

package:
  individually: true
  excludeDevDependencies: false

custom:
  bucket: stage.domain.com

provider:
  name: aws
  runtime: nodejs12.x
  region: ${opt:region, 'us-west-2'}
  stage: ${opt:stage, 'staging'}
  memorySize: 128
  timeout: 5
  iamRoleStatements:
    - Effect: Allow
      Action:
        - s3:*
      Resource: "*"

functions:
  resize:
    handler: handler.resize
    events:
      - s3:
          bucket: ${self:custom.bucket}
          event: s3:ObjectCreated:Put
          rules:
            - prefix: assets/
            - suffix: .png
          existing: true

When I deploy this function, I want dynamically set existing S3 bucket. How can I set multiple S3 buckets in serverless.yml to pick up at the time of deployment based on env ?