To setup composer on linux (im using debian), you need to have a ssh access or be present at the machine yourself.

Login into your SSH.

Use wget to download the most recent stable composer file (url is shown here: https://getcomposer.org/download/)
wget https://getcomposer.org/composer.phar
Then install composer into the system
mv composer.phar /usr/local/bin/composer

Now you should be able to simply use composer install or composer update and so on…
It might also work on the special linux that is included in windows 10, but i didn’t try it.