Updating dockerfile for release v1.2.0-rc1

docker/1.2/README.md

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+Mattermost
+==========
+
+http:/mattermost.org
+
+Mattermost is an open-source team communication service. It brings team messaging and file sharing into one place, accessible across PCs and phones, with archiving and search.
+
+Installing Mattermost
+=====================
+
+To run an instance of the latest version of mattermost on your local machine you can run:
+
+`docker run --name mattermost-dev -d --publish 8065:80 mattermost/platform`
+
+To update this image to the latest version you can run:
+
+`docker pull mattermost/platform`
+
+To run an instance of the latest code from the master branch on GitHub you can run:
+
+`docker run --name mattermost-dev -d --publish 8065:80 mattermost/platform:dev`
+
+Any questions, please visit http://forum.mattermost.org