Official image in dockerhub

[moonming]

https://docs.docker.com/docker-hub/official_images/
we need to add the official image of APISIX in dockerhub.

[hiproz]

1. i think we can ref from the alpine or centos version image of this repo
2. i think we shoud provide an official image that include the apisix etcd and dashbord

[hiproz]

i can try this work from the 1.1 version of apisix if that is possible :)

[moonming]

@hiproz The official image in dockerhub is here: https://hub.docker.com/r/apache/apisix, which is binding with this repo.

i think we shoud provide an official image that include the apisix etcd and dashbord

It's good for beginner, welcome PR.