Cleanup docs Dockerfile, so it matches best practices.

- Cleanup spacing throughout
- swap `&&` to beginning of lines to improve readability
- better cache busting for the SvenDowideit/mkdocs repo based on git commit id
- take advantage of the `WORKDIR`

Thanks to @tianon for the review so far :smile:.

I made this and realized I had included the same changes as @jfrazelle in #8715, but I think this adds a few more tweaks to make it more readable.
@SvenDowideit, How about this just before the `RUN` and after the current comment:
```markdown
# commit hash is the newest commit of SvenDowideit/mkdocs on
# docker-markdown-merge branch, it is used to break docker cache
# see: https://github.com/SvenDowideit/mkdocs/tree/docker-markdown-merge
```