Add deployment stage as an environment variable

template.yaml

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: hello_world/
      Handler: app.lambda_handler
      Runtime: python3.8
      Environment:
        Variables:
          # add a variable with the value of the DeploymentStage template parameter
          STAGE: !Ref DeploymentStage
          TABLE_NAME: !Ref MyTable
      Policies:
        - DynamoDBReadPolicy
            TableName: !Ref MyTable