Compress initramfs with zstd

See: https://gitlab.com/fedora/ostree/sig/-/issues/34

common.yaml

@@ -17,6 +17,8 @@ include:
   - sysroot-ro.yaml
   # Workaround for https://github.com/fedora-silverblue/issue-tracker/issues/587
   - grub2-workaround.yaml
+  # Compress initramfs with zstd
+  - initramfs-zstd.yaml
 
 # systemd-pcrphase module fails on ppc64le:
 # https://gitlab.com/fedora/ostree/sig/-/issues/44

initramfs-zstd.yaml

@@ -0,0 +1,21 @@
+# Initramfs compression with zstd
+packages:
+  - zstd
+
+postprocess:
+  - |
+    #!/usr/bin/env bash
+    set -xeuo pipefail
+
+    install -dm 0755 -o 0 -g 0 /usr/lib/dracut/dracut.conf.d
+
+  - |
+    #!/usr/bin/env bash
+    set -xeuo pipefail
+
+    cat > /usr/lib/dracut/dracut.conf.d/20-atomic-zstd.conf << 'EOF'
+    # 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.
+    compress="zstd -19 -q -T0"
+    EOF