When the project is hosted on Bitbucket Pipelines
The scripts supports a callback that will be called before
composer install is executed. This callback can be used to add credentials
to composer. To enable the callback go to Bitbucket Settings > Pipelines >
Environment Variables and add an environment variable called
COMPOSER_PRE_INSTALL_CALLBACK.
Example to add basic authentication for repo.example.com:
composer config --global http-basic.repo.example.com $YOUR_USER $YOUR_PASSWORD