Start toward testing s390x via cross

Author: EliahKagan

Cross.toml

@@ -0,0 +1,2 @@
+[target.s390x-unknown-linux-gnu]
+image = "cross-rs-gitoxide/s390x-unknown-linux-gnu"

etc/docker/Dockerfile.test-cross-s390x

@@ -0,0 +1,11 @@
+FROM ghcr.io/cross-rs/s390x-unknown-linux-gnu:latest
+
+RUN 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/*