Adds git to the Dockerfile apt install

I saw in setup.md "(if you have docker on Windows, I'd be grateful if
you could let me know if it works there too)".  So I went to build
it on Windows.

I use Windows Subsystem for Linux, which is (mostly) Ubuntu but not
really (there's no init).  But Docker Desktop has an integration for
WSL2.

For me, gradle wouldn't build the image without forcing git to install
in the container.  I'll attach screenshots to the pull request.  It just
looks like whatever ubuntu:focal my docker system grabbed doesn't have
git installed by default.

Anyway, this fixes it, and shouldn't screw anything even if git is
installed by default.  I tested it on my M1 Mac pro and it still works
fine.

Dockerfile

@@ -6,6 +6,7 @@ RUN apt update -y
 
 RUN apt install \
           openjdk-17-jdk \
+	  git \
           --assume-yes
 
 COPY . /code