build 2.5.1 with Alpine 3.6 due to sidekick Segfaults

[shleeable]

Hello,

Mastodon is built with sidekick and we're recently committed from ruby2.4.4-alpine3.6 to ruby2.5.1-alpine3.7... but have had to revert due to segfaults with sidekick.

We've planning to diagnose the root cause of these faults and would like to test with different version of alpine.

Thanks.

[shleeable]

mastodon/mastodon#8712

[tianon]

Mirroring my comment from #235:

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.