The defulat docker build (e.g. make emqx-docker) is based on alpine
image and it builds EMQ X from source code.
This is not flexible enough when we want to quickly run some tests
in a docker container.
The new docker build (e.g. make emqx-docker-testing) by default takes
the built zip package, and extract it in a very primitive base image
such as ubuntu:20.04 and centos:8
Zaiming Shi