Why does cloning a private git repository via Drush make file fail using SSH authentication? I have multiple private git accounts set up with SSH system's keychain authentication for each git repo.

It works to clone private git repos from command line:

git clone git@git_ssh_alias:git-account/some_repo.git


The git_ssh_alias is the alias name declared in .ssh/config file that specifies the SSH key that should be used.

However, this same clone doesn't work when running from make file via Drush script as following.

From Make file:

projects[theme_name][type] = "theme"
projects[theme_name][version] = 7.x-1.0
projects[theme_name][download][type] = "git"
projects[theme_name][download][url] = "git@git_ssh_alias:git-account/some_repo.git"
projects[theme_name][download][branch] = "master"


From Drush script:

drush_invoke_process('@self', 'make', array($make_file), $options)


where $make_file is a path to a make file and options are:

$options = array();
$options['no-core'] = TRUE;
$options['yes'] = TRUE;
$options['contrib-destination'] = 'sites/all';


My question: How to clone private git repo with local ssh keychain authentication enabled?
Using modules and not theme_name may resolve this.

projects[modules][type] = modules
projects[modules][download][type] = git
projects[modules][download][url] = git@bitbucket.org:company/repo.git
projects[modules][download][branch] = master


Here are a couple of useful links: 

https://groups.drupal.org/node/194303#comment-918073
http://www.systemseed.com/blog/drush-make-private-git-repository-github