added git binary on apk add for composer

[denistorresan]

Hi,
in some cases composer needs GIT to download a package (for example if package is hosted on github repo without packagist distribution).
In this case composer fails with this error:

1.617     Failed to download package from dist.
1.617     Now trying to download from source
1.874
1.877 In GitDownloader.php line 82:
1.877
1.877   git was not found in your PATH, skipping source download
1.877