Add Alpine variants for Drush.

[sylus]

Build passes locally but will need the following P.R. which adds alpine support to composer to pass in travis:

https://github.com/RobLoach/docker-composer/pull/56

Edit: P.R. is merged just waiting for docker tags.

I added postgres support as well since is what I needed, let me know. ^_^

I didn't add 6,7 since it seems they will be dropped soon anyways.

[sylus]

I had to add postgresql-dev which is a bit more heavy then i'd like because of:

https://bugs.alpinelinux.org/issues/3642

If you want me to remove the postgresql add-ons I can just use them in my sub-container so let me know ^_^.

[RobLoach]

This is great! :+1:

The base Composer images are still building in Docker Hub: https://hub.docker.com/r/composer/composer/builds/

Once that's through, mind seeing if the Travis tests pass?

[sylus]

Re-triggered and tests look good! :)

[RobLoach]

Thanks! :+1: