Add jq to container; simplify container build command

Installing `jq` in the container allows the `ask::askpass_only` and
`ask::username_password` to pass (they use `jq`).

Author: EliahKagan

etc/docker/Dockerfile.test-cross-s390x

@@ -8,6 +8,6 @@ RUN set -xC && \
     apt-key adv --keyserver keyserver.ubuntu.com \
         --recv-keys F911AB184317630C59970973E363C90F8F1B6217 && \
     apt-get update && \
-    apt-get install --no-install-recommends -y git && \
+    apt-get install --no-install-recommends -y git jq && \
     rm -rf /var/lib/apt/lists/* && \
     git config --system gitoxide.imaginary.arbitraryVariable arbitraryValue

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

@@ -1,5 +1,2 @@
 #!/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
+docker build -t cross-rs-gitoxide/s390x-unknown-linux-gnu - <etc/docker/Dockerfile.test-cross-s390x