installation doesn't see auth.json

[antwake]

I'm on MacOS Sierra 10.12
Composer is installed through brew
I can access composer globally

My installation settings are
Ubuntu 14
NGINX
PHP70
Magento2

my auth.json file contains the repo.magento.com Public and Private keys

cat .composer/auth.json 
{"http-basic": {"repo.magento.com": {"username": "5955bXXXXXXXXXX80543dcb", "password": "225XXXXXXXX7f"}}}

Yet the installation process asks me for my keys.

Looks like it can't read auth.json :
failed to open stream: No such file or directory /Users/patoche/.composer/vendor/richdynamix/magestead/src/Magestead/Helper/Options.php on line 201

If I echo the $authFile variable in Option.php, it shows /Users/patoche/.composer/auth.json
Which is where the file is located.

Strangely, if I hard-coded the path to the same value, it works
$authFile = '/Users/patoche/.composer/auth.json';

[richdynamix]

This is strange, I haven't encountered this one before. Strange it recognises the right file but can't access unless explicitly set. I'll investigate and get back to you.