Add system-scope config var in container

So installation_config tests can pass.

This also adjusts the RUN script in the dockerfile to show each
command before it runs and to make sure it is not overwriting
files unintentionally, and adds a convenience script outside for
building the image.

Author: EliahKagan

etc/docker/Dockerfile.test-cross-s390x

@@ -1,11 +1,13 @@
 FROM ghcr.io/cross-rs/s390x-unknown-linux-gnu:latest
 
-RUN apt-get update && \
+RUN set -xC && \
+    apt-get update && \
     apt-get install apt-transport-https && \
     echo 'deb https://ppa.launchpadcontent.net/git-core/ppa/ubuntu xenial main' > \
         /etc/apt/sources.list.d/git-core-ubuntu-ppa-xenial.list && \
     apt-key adv --keyserver keyserver.ubuntu.com \
         --recv-keys F911AB184317630C59970973E363C90F8F1B6217 && \
     apt-get update && \
     apt-get install --no-install-recommends -y git && \
-    rm -rf /var/lib/apt/lists/*
+    rm -rf /var/lib/apt/lists/* && \
+    git config --system gitoxide.imaginary.arbitraryVariable arbitraryValue

etc/docker/build-test-cross-s390x-image.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+set -e
+mkdir empty-context
+docker build -f etc/docker/Dockerfile.test-cross-s390x \
+    -t cross-rs-gitoxide/s390x-unknown-linux-gnu empty-context