It's deprecated for a while, reference:
- https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

Refactor Dockerfile, separate and move `apk add` upper

Reference - Best practices for writing Dockerfiles:

- https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#cmd

> CMD should almost always be used in the form of CMD ["executable","param1", "param2"…]

This will allow Docker to cache the pacakges apk added, which saves the
bandwidth and time to build this Docker image.

Fixes #270

Fixes #268