overlay: compress initrd with zstd -19 on FCOS and RHCOS/SCOS 9

On my system, relative to gzip -9, it roughly halves decompression time at
boot while also reducing initrd size.

RHCOS 8 can't inherit this because the RHEL 8 kernel doesn't enable
CONFIG_RD_ZSTD.

This requires coreos-installer to support decoding zstd (for
`coreos-installer pxe customize`), which is
coreos/coreos-installer#920.

For coreos/fedora-coreos-tracker#1247.

overlay.d/06el9/usr/lib/dracut/dracut.conf.d/coreos-zstd.conf

@@ -0,0 +1,8 @@
+# Compress initrd with zstd.  dracut defaults to -15, but we want the
+# maximum reasonable compression, so override the command line to use
+# dracut's defaults along with -19.
+#
+# We can't use this in RHCOS 8 because the kernel doesn't enable
+# CONFIG_RD_ZSTD.
+
+compress="zstd -19 -q -T0"

tests/kola/files/initrd/compression

@@ -0,0 +1,20 @@
+#!/bin/bash
+## kola:
+##   exclusive: false
+#
+# If dracut can't find the configured compressor, it warns and falls back to
+# gzip (!).  Fail if the initrd isn't compressed with zstd.
+
+set -xeuo pipefail
+
+. $KOLA_EXT_DATA/commonlib.sh
+
+# Check initrd for zstd magic number
+if is_rhcos8; then
+    ok "Skipping initrd zstd compression test on RHCOS 8"
+else
+    if ! LANG=C grep -aUPq "\x28\xb5\x2f\xfd" /boot/ostree/*/init*; then
+        fatal "Didn't find zstd compression in initrd"
+    fi
+    ok "Found zstd compression in initrd"
+fi