Deploy to Heroku using the container registry

[lfborjas]

References:

• https://devcenter.heroku.com/articles/container-registry-and-runtime
• https://github.com/heroku/alpinehelloworld/blob/master/Dockerfile
• https://github.com/cdepillabout/servant-on-heroku/blob/master/Dockerfile (from: https://arow.info/blog/posts/2017-03-30-servant-on-heroku.html, which is a bit too verbose -- things have gotten better in the past 3 years it seems like.)
• https://www.fpcomplete.com/blog/2017/12/building-haskell-apps-with-docker/
• Using this base image for building: https://hub.docker.com/_/haskell
• https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#use-multi-stage-builds
• https://devcenter.heroku.com/articles/pipelines

See notes in the individual commits, but some caveats:

• Had to update the stack resolver, as the haskell:8 image in Docker didn't have ghc 8.8.3 (only 8.8.4)
• Had to use debian, as trying to use alpine kept giving me file not found errors -- surely due to some lower-level dependency? If we figure out how to use alpine, the final image should be ~50MB (vs. the current ~160MB!)
• Had to use only 1 job for the stack build, as any parallelization, if the benighted haskell-src-exts (a transitive dependency of interpolate, via haskell-src-meta) got to build in parallel with something else big (maybe happy,) the build died -- one can also give Docker more memory, but 2GB and 3 cores should be enough! (I have no problem in my 8 core/8 GB computer, but this is a tiny app!) -- using only one thread isn't too bad: a build from scratch takes ~11 minutes.