Add ssh to docker container?

Question

Docker

Hello! I have a docker-compose in which a php container is added.
How do I correctly add the ssh key from my pc to the container so that I can install composer install?

version: "3.7"

services:
  php:
    container_name: 'php'
    image: digitalocean.com/php
    build:
      context: ./php
      dockerfile: Dockerfile
    volumes:
      - ../:/project
      - ./php/supervisor/conf.d:/etc/supervisor/conf.d/:ro
    working_dir: /project
    networks:
      - lemp