Update to Alpine 3.7

https://www.alpinelinux.org/posts/Alpine-3.7.0-released.html

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.6
+FROM alpine:3.7
 MAINTAINER searx <https://github.com/asciimoo/searx>
 LABEL description "A privacy-respecting, hackable metasearch engine."