Add support for alpine linux 3.x. Closes #16
Add support for alpine linux 3.x. Closes #16

Alpine Linux 3.x uses different flags for the `adduser` call. Specifically, the following map to their debian counterparts:

- -D: `--disabled-password`
- -g: `--gecos`
- -s: login shell (none by default)

We also need to set the password to none manually

dokku/sshcommand,progrium/sshcommand