Dockerfile: Upgrade the version of git bundled in the docker image

This fixes the root cause for PR #210, differently.

Dockerfile

@@ -17,6 +17,21 @@ RUN cd /opt && \
     mv cmake-* cmake
 ENV PATH=/opt/cmake/bin:$PATH
 
+# Install a newer version of Git; the version of Git in Ubuntu 18.04 is
+# said to have issues with submodules, see e.g.
+# https://github.com/mstorsjo/llvm-mingw/pull/210#issuecomment-870104971 and
+# https://github.com/mstorsjo/llvm-mingw/pull/210#issuecomment-873486503.
+# This isn't needed for building LLVM itself, but makes the built Docker
+# image more useful for use as image for building other projects. If updating
+# to a newer distribution, this can be dropped.
+RUN apt-get update -qq && \
+    apt-get install -qqy --no-install-recommends software-properties-common && \
+    add-apt-repository ppa:git-core/ppa && \
+    apt-get update -qq && \
+    apt-get upgrade -qqy git && \
+    apt-get clean -y && \
+    rm -rf /var/lib/apt/lists/*
+
 
 RUN git config --global user.name "LLVM MinGW" && \
     git config --global user.email root@localhost