Upgrade s6-overlay from 2.2.0.3 to 3.1.2.1 (#4584)

Also remove the unnecessary xz-utils from the final image.

docker/rootfs/etc/services.d/nginx/finish

@@ -1,7 +1,7 @@
-#!/usr/bin/with-contenv bash
+#!/command/with-contenv bash
 # shellcheck shell=bash
 # Take down the S6 supervision tree when the process fails
 
 if [[ "${1}" -ne 0 && "${1}" -ne 256 ]]; then
-  exec s6-svscanctl -t /var/run/s6/services
+  exec /run/s6/basedir/bin/halt
 fi

docker/rootfs/etc/services.d/nginx/run

@@ -1,4 +1,4 @@
-#!/usr/bin/with-contenv bash
+#!/command/with-contenv bash
 # shellcheck shell=bash
 
 exec nginx