Update the alpine base image

[jehrhardt]

The alpine base image is already available in version 3.5. The ruby image should also use this version.

[tianon]

See also #103, #108, and docker-library/golang#131 (comment). 😞

[jehrhardt]

Does it stay open until the Go update is done?

[nilbus]

@jehrhardt Would you like to bump this to alpine:3.6?

[nilbus]

@tianon It looks like we have 2 options on the table to move forward—update the existing alpine tag and potentially break backward compatibility, or follow the example of docker-library/golang and create a new -alpine3.6 tag. Who can make that decision for this repo?

[tianon]

There are only two ways we're getting Alpine 3.6 for this repo, given that the common usage of it is FROM ruby:xxx in a user's Dockerfile:

1. a new Ruby major release
2. a new alpine3.6 variant (as has been done elsewhere)

Since Ruby 2.5 probably isn't right around the corner, we should move ahead with adding an alpine3.6 variant to Ruby 2.4. See docker-library/golang#162 for an example of how this was done for the golang image.

@jehrhardt would you like to update this PR / make a new one to implement an alpine3.6 variant for Ruby 2.4, or would you prefer that I take over from here?

[ojab]

I assume this PR can be closed?

[nilbus]

@ojab Yes, I think this is safe to close.
@tianon Would you mind taking over in a new PR, considering @jehrhardt hasn't been able to respond?

[tianon]

Doh! Already done in #145. 👍

[nilbus]

awesome. Thanks!​