Add gpg to the image and optionally create a keyring

If the ARCHISO_GNUPG_FD environment variable is set, its contents will be interpreted as an open file
descriptor and its contents will be used to create a keyring in the initramfs in /gpg.

install/archiso

@@ -14,11 +14,16 @@ build() {
     add_binary losetup
     add_binary mountpoint
     add_binary truncate
+    add_binary gpg
 
     add_file /usr/lib/udev/rules.d/60-cdrom_id.rules
     add_file /usr/lib/udev/rules.d/10-dm.rules
     add_file /usr/lib/udev/rules.d/95-dm-notify.rules
     add_file /usr/lib/initcpio/udev/11-dm-initramfs.rules /usr/lib/udev/rules.d/11-dm-initramfs.rules
+    if [[ $ARCHISO_GNUPG_FD ]]; then
+        mkdir -p "$BUILDROOT$dest"/gpg
+        gpg --homedir "$BUILDROOT$dest"/gpg --import <&$ARCHISO_GNUPG_FD
+    fi
 }
 
 # vim: set ft=sh ts=4 sw=4 et: