badge with latest semver tag on docker hub (Overv#262)

README.md

@@ -1,6 +1,7 @@
 # openstreetmap-tile-server
 
 [![Build Status](https://travis-ci.org/Overv/openstreetmap-tile-server.svg?branch=master)](https://travis-ci.org/Overv/openstreetmap-tile-server) [![](https://images.microbadger.com/badges/image/overv/openstreetmap-tile-server.svg)](https://microbadger.com/images/overv/openstreetmap-tile-server "openstreetmap-tile-server")
+[![Docker Image Version (latest semver)](https://img.shields.io/docker/v/overv/openstreetmap-tile-server?label=docker%20image)](https://hub.docker.com/r/overv/openstreetmap-tile-server/tags)
 
 This container allows you to easily set up an OpenStreetMap PNG tile server given a `.osm.pbf` file. It is based on the [latest Ubuntu 18.04 LTS guide](https://switch2osm.org/serving-tiles/manually-building-a-tile-server-18-04-lts/) from [switch2osm.org](https://switch2osm.org/) and therefore uses the default OpenStreetMap style.