重新定义 build bookdown，Docker 搭配 Travis 更佳哦

2020-08-23 前言：在维持开发环境一致性方面，Travis 确实做得很好，但还是 Docker 更彻底，debug 时能保证 local 和 server 完全一样。

build bookdown 更改实现方式，用了 Docker 之后就优雅多了（-w 任我行），不用再像 Travis 那样担心 working directory。附 Travis 实现的逻辑：

The biggest restraint is that bookdown::render_book() requires workspace at the book folder.

For blogdown, .Rmd lives in docs/. If you setwd() to there, many paths would be relative to it (like ../../output, ../../extra/_output.yaml).

So I thought about adding a environment variable input_sub_dir. But blogdown doesn’t collaborate, it refer to the some source code of the package by ../R/***.R.

Suddenly, I realized that instead of setting every path relative to workspace, we can have to_work_dir to walk to $work_dir. In this way, every other path remains unchanged (output, extra/_output.yaml), user only need to customize rmd_dir & to_work_dir, like rmd_dir=.; to_work_dir=..;, rmd_dir=docs; to_work_dir=../..;.