Update BestPractices.md with alpine user
Update BestPractices.md with alpine user

Added entry to **Non-root User** section to show adding user & group in the alpine variant since the commands are a little different.
chorrell/docker-node,joyent/docker-node,chorrell/docker-node,sirgallifrey/docker-node-test,nodejs/docker-node,nodejs/docker-node,pesho/docker-node