How to include private repo via ssh using Composer from Docker

I'm using Mac and Docker for my local PHP/Nginx environment.

I decided to add my own private repository to composer.json. It looks something like this:

"repositories":[
        {
            "type": "vcs",
            "url": "ssh://git@privaterepo.com/my-repo/php-code-fixer.git"
        }
    ]

I tried to execute following command: docker exec -it mysite_php_1 php /usr/local/bin/composer.phar require --dev my-repo/php-code-fixer

I get this error:

 [RuntimeException]                                                                                       
  Failed to execute git clone --mirror 'ssh://git@privaterepo.com/my-repo/php-code-fixer.git'  
   '/root/.composer/cache/vcs/ssh---ssh://git@privaterepo.com/my-repo/php-code-fixer.git/'           
                                                                                                           
  Cloning into bare repository '/root/.composer/cache/vcs/ssh---ssh://git@privaterepo.com/my-repo/php-code-fixer.git'...                                                                                    
  git@privaterepo.com: Permission denied (public key).                                              
  fatal: Could not read from remote repository.                                                            
                                                                                                           
  Please make sure you have the correct access rights                                                      
  and the repository exists.  

I tried to clone the repo via ssh without docker and it works. I have permission.

I tried to solve above problem by mounting my .ssh folder to docker container like this:

  php:
    volumes:
      - ~/.ssh:/root/.ssh

but then I get

[RuntimeException]                                                                                       
  Failed to execute git clone --mirror 'ssh://git@privaterepo.com/my-repo/php-code-fixer.git'  
   '/root/.composer/cache/vcs/ssh---ssh://git@privaterepo.com/my-repo/php-code-fixer.git/'           
                                                                                                           
  Cloning into bare repository '/root/.composer/cache/vcs/ssh---my ssh repo'...                                                                                    
  /root/.ssh/config: line 8: Bad configuration option: usekeychain                                         
  /root/.ssh/config: terminating, 1 bad configuration options                                              
  fatal: Could not read from remote repository.                                                            
                                                                                                           
  Please make sure you have the correct access rights                                                      
  and the repository exists. 

How do I install my private repository using Composer from Docker? I cannot execute composer outside of docker because my machine doesn't have Memcached installed (the docker environment does) and because of that I'm getting Class 'Memcached' not found. I could install Memcached but then everyone who wants to use my script needs to have Memcached installed.