link github token into container

[ivoba]

Currently i have to add to each setup a new github token:

Could not fetch https://api.github.com/repos/xxx/DoctrineExtensions/commits/xxx, please create a GitHub OAuth token to go over the API rate limit
Head to https://github.com/settings/tokens/new?scopes=repo&description=Composer+on+docker+setup1
to retrieve a token. It will be stored in "/var/www/auth.json" for future use by Composer.

we should link the host's /var/www/auth.json to the container.

[gherkins]

yep, also had to deal with this...
maybe with an exists check?