Run stack build with only one job

It seems that certain haskell packages (in my case, haskell-src-exts,)
take up so much memory that docker dies trying to run them, so we make
it run with one job, vs. as many as possible in parallel.

See:

* https://github.com/facebook/duckling/blob/7889f396f324028075d39e5003f1083ad05d68f3/Dockerfile
* facebook/duckling#203
* wireapp/wire-server#562
* https://stackoverflow.com/questions/56496852/problem-building-a-docker-container-with-haskell-stack-how-can-i-ensure-that-ha

Dockerfile

@@ -8,7 +8,7 @@ WORKDIR /opt/freenatalchart
 
 # Install dependencies and move binary to `/bin`
 RUN stack --system-ghc build -j1 servant diagrams
-RUN stack --system-ghc build
+RUN stack --system-ghc build -j1
 RUN stack --local-bin-path /opt/freenatalchart/bin install
 
 # Add user and setup path (for local testing, ignored by Heroku)