Bump Alpine base image version to v3.8 [skip ci]

PeterDaveHello · PeterDaveHello · commit 0f76c7a07e86 · 2018-07-06T22:55:15.000+08:00
diff --git a/config b/config
@@ -1,3 +1,3 @@
 baseuri     https://nodejs.org/dist
 default_variant jessie
-alpine_version  3.7
+alpine_version  3.8
