[mod] Dockerfile: upgrade to Alpine 3.13

See:
* https://alpinelinux.org/posts/Alpine-3.13.0-released.html
* musl>=1.2.1 have a new malloc implementation (mallocng) :

Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.12
+FROM alpine:3.13
 ENTRYPOINT ["/sbin/tini","--","/usr/local/searx/dockerfiles/docker-entrypoint.sh"]
 EXPOSE 8080
 VOLUME /etc/searx