How to define build-args in docker-compose?

You can specify the arguments directly in your docker-compose file:

node1:
    build:
        context: node1
        args:
            ADMIN_USERNNAME: weblogic1
            ADMIN_PASSWORD: weblogic1
            ADMIN_NAME: admin1
    image: node1
    container_name: node1

See a full example: MWE

The official docs have all detail.