[Ur] Dockerfile for building Ur/web apps + running on Heroku

[Oisín Mac Fhearaí]

While playing around with Heroku, I decided to try their new-ish ability to
build and deploy apps from a Dockerfile, rather than the buildpack method
which has some limitations. Also, I wanted to be able to run a recent
version of Ur/web.

First, I created a Docker image with Ur/web built from master, with
dependencies installed, and pushed it to Docker hub:

https://hub.docker.com/r/destynova/urweb

Then I made a small app that includes a heroku.yml and a Dockerfile that
extends the compiler image in Docker Hub, so pushing with "git push heroku
master" triggers the build and deploy. It takes about 70 seconds for me.
It's a very simplistic app with no database interaction (I configured it to
use a dummy SQLite database with no content). The source is here:

https://github.com/DestyNova/memvalid

It's running on a free Heroku dyno and available here for the time being:

https://memvalid.herokuapp.com/

Might be handy if you're thinking of trying Heroku or any other system that
can deploy from a Docker image.

Oisín

PS. I also noticed while writing the app that Ur/web seems to be lacking
some basic string-handling functions. To get the basic feature of the app
working, I implemented a couple of them that might be good to have in the
stdlib, or maybe could live as a standalone library (I don't know exactly
how this is managed in Ur/web). For example:

fun explode (txt: string) : list char
fun tokens (txt: list char) (p: char -> bool) : list string
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