Optimize Dockerfile

[PeterDaveHello]

This will make the image much smaller!

REPOSITORY     TAG                  IMAGE ID       CREATED          SIZE
after          latest               517f10c93ee7   17 minutes ago   1.21GB
before         latest               9a4cda688b4b   5 hours ago      1.31GB

[EverettBerry]

Thank you sir. Need to merge this and 2 other docker prs.

[PeterDaveHello]

Interesting, just took a look at another two PRs, looks like that's something I'm planning but haven't done yet, but somehow part of them is now overlapped. BTW, this tool is quite useful, another Docker related optimization is currently preparing, and should better wait for all these existing ones being merged first.