Add user support to dockerfile

[HackToHell]

This allows emqtt to run on platform such as openshift without changes to the dockerfile

Fixes #15

Make sure all boxes are checked (add x inside the brackets) when you submit your contribution, remove this sentence before doing so.

• [*] I have thoroughly tested my contribution.
• [*] The code changes are reflected in the documentation README.md.

[emqplus]

@HackToHell Thanks for your contributions.