Update Dockerfile (#127)

According to the Dockerfile reference, MAINTAINER has been deprecated
in favour of using the LABEL instruction. This patch updates the file
to use that instruction instead.

Fixes #126.

2.3.0/Dockerfile

diff --git a/2.3.0/Dockerfile b/2.3.0/Dockerfile
index fcf175b..ad1d3d3 100644
--- a/2.3.0/Dockerfile
+++ b/2.3.0/Dockerfile
@@ -12,7 +12,7 @@
 
 FROM debian:stretch-slim
 
-MAINTAINER CouchDB Developers dev@couchdb.apache.org
+LABEL maintainer="CouchDB Developers dev@couchdb.apache.org"
 
 # Add CouchDB user account to make sure the IDs are assigned consistently
 RUN groupadd -g 5984 -r couchdb && useradd -u 5984 -d /opt/couchdb -g couchdb couchdb