Dockerfile: add git package to the image

This enables code to be checked out from within the running container as is
typically used by CI runners.

Dockerfile

@@ -7,7 +7,7 @@ RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y \
         cmake libgmp-dev libmpfr-dev libmpc-dev \
         libboost-all-dev bison texinfo \
-        ruby flex curl g++
+        ruby flex curl g++ git
 
 # Build image
 FROM base AS build