Use new LABEL syntax for Dockerfile

https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

Dockerfile

 FROM alpine:3.6
-MAINTAINER searx <https://github.com/asciimoo/searx>
-LABEL description "A privacy-respecting, hackable metasearch engine."
+LABEL maintainer="searx <https://github.com/asciimoo/searx>"
+LABEL description="A privacy-respecting, hackable metasearch engine."
 
 ENV BASE_URL=False IMAGE_PROXY=False
 EXPOSE 8888