docker: update usage

[noraj]

WebMap/docker/Dockerfile

Lines 7 to 12 in fc28245

	# Usage:
	# $ cd /opt
	# $ git clone https://github.com/Hamza-Megahed/WebMap.git
	# $ cd WebMap/docker
	# $ docker build -t webmap:latest .
	# $ docker run -d -v /opt/WebMap/docker/xml:/opt/xml -p 8000:8000 webmap:latest