matrix: added main page and docker image page

wiki/linux/mkinitcpio.md

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+# mkinitcpio
+
+`mkinitcpio` is a script to create an initial ramdisk environment.
+
+## Troubleshooting
+
+### '/lib/modules/xxxxxxx' is not a valid kernel module directory
+
+When you encounter an error like the header of this section when running
+`mkinitcpio` try first to reinstall the `linux`, `linux-headers` and
+`filesystem` packages.
+If that does not fix it continue.
+Look into the `/lib/modules` directory.
+There should be a directory called `4.9.8-1-ARCH` or similar.
+Now run `mkinitcpio -k 4.9.8-1-ARCH -g /boot/image.img` and move the image
+to the correct location `/boot/initramfs-linux.img` in the boot directory (back
+up the old one).
+After that reboot and it should again work fine (Source:
+[Unix Stackexchange](https://unix.stackexchange.com/questions/395402/lib-modules-4-9-8-1-arch-is-not-a-valid-kernel-module-directory)).