How about add env_example file ?

Looks we have [build.sh](https://github.com/hyperf/hyperf-docker/blob/master/build.sh) file can build/check all type of images.
But actual users may only want to build one image.
Add `env_example` may helpful for that case.

Start by reading build.sh to understand how it builds and checks all image types, then identify the inputs needed to build a single image. Confirm which variables belong in the proposed env_example file and whether the example should be consumed by build.sh; the work is done when users can follow it to build one image.

Written by the indexing model from the issue text.