Install and use tini

Using `python` as PID 1 like this doesn't actually work because PID 1
doesn't have any default signal handlers.  Thus, when the container
runtime tries to stop the container by sending SIGTERM to the main
process, nothing happens and it eventually has to send SIGKILL to stop
it.  By using a "real" init process as PID 1, we can be sure that signal
handlers are set up correctly, plus, we won't leave a bunch of zombie
processes while the container is running.

Containerfile

@@ -1,10 +1,13 @@
 FROM registry.fedoraproject.org/fedora:latest
 
-RUN groupadd -g 1000 jenkins \
+RUN --mount=type=cache,target=/var/cache \
+    dnf install -y \
+        tini \
+    && groupadd -g 1000 jenkins \
     && useradd -c 'Jenkins user' -g 1000 -l -M -s /bin/sh -u 1000 jenkins
 
 COPY dch-root-ca.crt /etc/pki/ca-trust/source/anchors/
 
 RUN update-ca-trust
 
-CMD ["python3", "-c", "import signal;signal.pause()"]
+CMD ["tini", "sleep", "--", "infinity"]