Manually set up PPA + other fixups

This avoids installing `software-properties-common` in the
container, instead manually setting up the PPA again. Accordingly,
it is not necessary to uninstall any packages to avoid high bloat
in the image, so that is removed. Other cleanup is still done.

This also removes the old `git` and packages that were marked as
automatically installed that were only needed as its dependencies,
and shows details about the `git` that is installed.

For s390x, this works to cause the test suite to use a s390x build
of `git`, at a reasonably new version as provided by the PPA. That
should happen for some other targets, but not all. This therefore
shows the `git` version as well as examining the executable to
report its architecture while the container is being built.

Note that this does not yet appear sufficient to produce the s390x
test failures mentioned in #1687 and discused in #1698 comments.
(That might relate to how this is still just testing `max-pure`.)

Author: EliahKagan

etc/docker/Dockerfile.test-cross

@@ -11,17 +11,23 @@ RUN set -uC && \
         apt_suffix=":$target_arch"; \
     fi && \
     apt-get update && \
-    apt-get install --no-install-recommends -y software-properties-common && \
-    add-apt-repository -y ppa:git-core/ppa && \
+    apt-get install --no-install-recommends -y apt-transport-https gnupg && \
+    release="$(sed -n 's/^VERSION_CODENAME=//p' /etc/os-release)" && \
+    echo "deb https://ppa.launchpadcontent.net/git-core/ppa/ubuntu $release main" \
+        >/etc/apt/sources.list.d/git-core-ubuntu-ppa.list && \
+    apt-key adv --keyserver keyserver.ubuntu.com \
+        --recv-keys F911AB184317630C59970973E363C90F8F1B6217 && \
     apt-get update && \
+    apt-get purge --autoremove -y git && \
     apt-get install --no-install-recommends -y "libc6$apt_suffix" "libcurl3-gnutls$apt_suffix" \
         "libexpat1$apt_suffix" "libpcre2-8-0$apt_suffix" "zlib1g$apt_suffix" perl liberror-perl \
-        git-man patch jq && \
+        git-man file jq patch && \
     apt-get download "git$apt_suffix" && \
     dpkg --ignore-depends="perl$apt_suffix,liberror-perl$apt_suffix" -i git[-_]*.deb && \
-    echo INSTALLED/UPGRADED GIT && \
+    git version --build-options && \
+    git="$(command -v git)" && \
+    file -- "$git" && \
     rm git[-_]*.deb && \
-    apt-get purge --autoremove software-properties-common && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/* && \
     if test -f /android-runner; then \