Update base image from Bionic to Disco

[raxod502]

This is needed for Python 3.7, which we are upgrading Repl.it to use. Also:

• Install Emacs 26 from official repos instead of PPA.
• Install Julia to /usr/local instead of /usr, and upgrade from 1.1.0 to 1.1.1.
• No longer install m4 and opam.
• Add explanatory comments to Docker install script.

[raxod502]

Needs to be merged with https://github.com/replit/goval/pull/768.

[replbot]

unbooping: approved