Versioning improvements (#768)

Co-authored-by: ReenigneArcher <42013603+ReenigneArcher@users.noreply.github.com>

docker/ubuntu-20.04.dockerfile

@@ -14,6 +14,15 @@ FROM sunshine-base as sunshine-build
 ARG TARGETPLATFORM
 RUN echo "target_platform: ${TARGETPLATFORM}"
 
+ARG BRANCH
+ARG BUILD_VERSION
+ARG COMMIT
+# note: BUILD_VERSION may be blank
+
+ENV BRANCH=${BRANCH}
+ENV BUILD_VERSION=${BUILD_VERSION}
+ENV COMMIT=${COMMIT}
+
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 # install dependencies
 RUN <<_DEPS
@@ -24,6 +33,7 @@ apt-get install -y --no-install-recommends \
   build-essential=12.8* \
   gcc-10=10.3.0* \
   g++-10=10.3.0* \
+  git=1:2.25.1* \
   libavdevice-dev=7:4.2.* \
   libboost-filesystem-dev=1.71.0* \
   libboost-locale-dev=1.71.0* \