Cannot Create container source is not directory. Encountered error while bringing up the project

I have written dockerfile like below:

FROM nginx:1.13.8-alpine
RUN apk add --no-cache curl

COPY app/index.php /app

COPY nginx/default.conf /etc/nginx/conf.d/default.conf

And Docker compose file like below:

version : '3.3'

services: 
    php:
      build: ./php/
      volumes:
       - 'appvolume:/app'
       - '/php/local.ini:/usr/local/etc/php/conf.d/local.ini'
      working_dir: /app
      container_name: php_cli
#mysql service
    mysql:
        image: mysql:8.0
        container_name: mysql-server-80
        command : --default-authentication-plugin=mysql_native_password
        working_dir: /app
        volumes:
          - 'dbdata:/var/lib/mysql'
        restart: always
        environment:
          MYSQL_ROOT_PASSWORD : 'duet365'
          MYSQL_DATABASE : 'world'
          MYSQL_USER : 'db_user'
          MYSQL_PASSWORD : 'mypwd'
          SERVICE_NAME: 'mysql'
        ports:
          - '9000:3306' 
        expose:
          - '3306' 
    app:
      build: ./
      volumes:
        - 'appvolume:/app'
      ports:
      - '4000:4000'
      - '444:443'
      links:
        - mysql 
      depends_on:
        - php
        - mysql
#Volumes
volumes:
  dbdata:
    driver: local
  appvolume:

My project folder is like this:
php web app docker project folder

After running the command docker-compose up I am getting following error:

ERROR: for 8a0cce0f4dbe_php_docker_app_1  Cannot create container for service app: source is not directory

ERROR: for app  Cannot create container for service app: source is not directory
ERROR: Encountered errors while bringing up the project.

As I am new in docker, I am not getting where is the mistake. As far I know COPY command is all right.

Source: Docker Questions