Append ruby 2.5 on Alpine 3.6

[shleeable]

Closes #233

[tianon]

I'm not sure how I feel about adding a new variant based on Alpine 3.6 (especially as we're trying to move forward into Alpine 3.8 to replace 3.7 by default) -- I think the release last week (for the apk security fix) was more the exception than the norm, and that Alpine 3.6's ship has really sailed generally except for severe security vulnerabilities.

[shleeable]

using ruby 2.5.1 for example..

• alpine 3.6 doesn't exist
• alpine 3.7 crashes
• alpine 3.8 can't exist (pending next release)

There isn't really much of an option for me.. Just don't use 2.5?

[tianon]

Do you know why it crashes on 3.7? I don't recall seeing any other reports of issues like that with that combination, so it seems like an isolated case and it would be useful if we could determine why it's happening and whether there's anything we can do.

[shleeable]

I'm happy to assist to put the pieces together... booting 2.5.1 on alpine 3.6 was actually just step 1 of isolating if it's Ruby or alpine :/

I've created an issue ticket - this PR is not blocked to my crash :)