Add support for Alpine 3.7, 3.6, 3.5 and 3.4

Let's add support for the most recent Alpine versions:
- 3.7, released 2017-11-30, stable
- 3.6, released 2017-05-24, old stable
- 3.5, released 2016-12-22, old old stable (limited support, for system images only)
- 3.4, released 2016-05-31, old old stable (limited support, for system images only)