Docker Error: /usr/local/bin/docker-php-entrypoint: exec: line 9: /usr/bin/supervisord: not found

I have docker image websocket.dockerfile like below

FROM php:7-cli-alpine

RUN apk --update add supervisor

RUN rm /var/cache/apk/* && \
    mkdir -p /var/www

COPY websockets.conf /etc/supervisor/supervisord.conf

ENTRYPOINT ["/usr/bin/php", "/var/www/html/websocket-service/artisan", "websockets:serve"]

below websocket.conf

[supervisord]
nodaemon=true

[program:websockets]
command=/usr/bin/php /var/www/html/websocket-service/artisan websockets:serve
numprocs=1
autostart=true
autorestart=true
user=laravel-echo

Below docker-compose.yml

version: '2'

services:
   websockets:
    build:
      context: ./
      dockerfile: websocket.dockerfile
    volumes:
      - ./:/var/www/html
    ports:
      - 6001:6001

When i run docker-compose up command it shows me below error and doesn't run ENTRYPOINT command. I want it to run that command.

websockets_1  | /usr/local/bin/docker-php-entrypoint: exec: line 9: /usr/bin/supervisord: not found
websocket-service_websockets_1 exited with code 127

What is wrong i have done?