Rebase on scratch, update to v0.21.0

There's no reason we need anything from the _rsync_ container image to
use Zola.  Build pipelines that need to do more than run `zola build`
can use different containers to access those tools.

Containerfile

@@ -1,7 +1,8 @@
-FROM git.pyrocufflink.net/containerimages/rsync
+FROM scratch
 
-COPY --from=ghcr.io/getzola/zola:v0.16.1 /bin/zola /usr/local/bin/zola
+COPY --from=ghcr.io/getzola/zola:v0.21.0 /bin/zola /bin/zola
+COPY --from=registry.k8s.io/pause:3.9 /pause /bin/pause
 
-USER jenkins
+ENV PATH=/bin
 
-CMD ["/bin/python3", "-c", "import signal; signal.pause()"]
+CMD ["pause"]

Jenkinsfile

@@ -1 +1 @@
-buildContainerImage()
+buildContainerImage2(archlist: ["amd64","arm64"])