Reduces image size using Alpine + Multistage (re: #278)

[iMerica]

Hey Simon!

I got the image size down from 256MB to 110MB.

Seems to be working okay, but you might want to test it a bit more.

Example output of docker run --rm -it <my-tag> datasette

Serve! files=() on port 8001
[2018-05-23 05:23:08 +0000] [1] [INFO] Goin' Fast @ http://127.0.0.1:8001
[2018-05-23 05:23:08 +0000] [1] [INFO] Starting worker [1]

Related: #278

[simonw]

I'm afraid I just merged #280 which means this no longer applies. You're very welcome to see if you can further optimize the new Dockerfile though.