Fix runners to do their best to gracefully stop on pod eviction (#1759)

Ref #1535
Ref #1581

Signed-off-by: Yusuke Kuoka <ykuoka@gmail.com>

runner/entrypoint-dind-rootless.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+source logger.sh
+source graceful-stop.sh
+trap graceful_stop TERM
+
+log.notice "Writing out Docker config file"
+/bin/bash <<SCRIPT
+mkdir -p /home/runner/.config/docker/
+
+if [ ! -f /home/runner/.config/docker/daemon.json ]; then
+  echo "{}" > /home/runner/.config/docker/daemon.json
+fi
+
+if [ -n "${MTU}" ]; then
+jq ".\"mtu\" = ${MTU}" /home/runner/.config/docker/daemon.json > /tmp/.daemon.json && mv /tmp/.daemon.json /home/runner/.config/docker/daemon.json
+# See https://docs.docker.com/engine/security/rootless/ and https://github.com/docker/engine/blob/8955d8da8951695a98eb7e15bead19d402c6eb27/contrib/dockerd-rootless.sh#L13
+echo "DOCKERD_ROOTLESS_ROOTLESSKIT_MTU=${MTU}" >> /etc/environment
+fi
+
+if [ -n "${DOCKER_REGISTRY_MIRROR}" ]; then
+jq ".\"registry-mirrors\"[0] = \"${DOCKER_REGISTRY_MIRROR}\"" /home/runner/.config/docker/daemon.json > /tmp/.daemon.json && mv /tmp/.daemon.json /home/runner/.config/docker/daemon.json
+fi
+SCRIPT
+
+log.notice "Starting Docker (rootless)"
+
+dumb-init bash <<'SCRIPT' &
+# Note that we don't want dockerd to be terminated before the runner agent,
+# because it defeats the goal of the runner agent graceful stop logic implemenbed above.
+# We can't rely on e.g. `dumb-init --single-child` for that, because with `--single-child` we can't even trap SIGTERM
+# for not only dockerd but also the runner agent.
+/home/runner/bin/dockerd-rootless.sh --config-file /home/runner/.config/docker/daemon.json >> /dev/null 2>&1 &
+
+startup.sh
+SCRIPT
+
+RUNNER_INIT_PID=$!
+log.notice "Runner init started with pid $RUNNER_INIT_PID"
+wait $RUNNER_INIT_PID
+log.notice "Runner init exited. Exiting this process with code 0 so that the container and the pod is GC'ed Kubernetes soon."
+
+trap - TERM