Add doc how to build a registry image with VM disk

doc/image-from-registry.md

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+# Creating a registry image with a VM disk
+The purpose of this document is to show how to create registry image containing a Virtual Machine image that can be imported into a PV.
+
+## Prerequisites
+The VM disk image should be called 'disk.img' and placed in the root folder.
+
+## Create an image with Buildah
+
+Buildah is a tool that facilitates building Open Container Initiative (OCI) container images.
+
+More information is available here: [Buildah tutorial](https://github.com/containers/buildah/blob/master/docs/tutorials/02-registries-repositories.md).
+
+Create a new container image from scratch and keep its name in a variable:
+
+```bash
+$ newcontainer=$(buildah from scratch)
+```
+
+Add the VM disk to the container image and commit:
+
+```bash
+$ buildah copy $newcontainer ./disk.img /
+buildah commit $newcontainer my-vm-img
+```
+
+To push an image from your local Buildah container storage, check the image name, then push it using the buildah push command. Remember to identify both the local image name and a new name that includes the location (<registry>:5000, in this case):
+```
+buildah push --tls-verify=false my-vm-img:latest <registry>:5000/my-vm-img:latest
+```
+
+## Create an image with Docker
+
+Create a Dockerfile with the following content:
+
+```
+FROM scratch
+ADD disk.img /
+```
+
+Build, tag and push the image:
+
+```bash
+$ docker build . -t my-vm-img
+$ docker tag my-vm-img <registry>:5000/my-vm-img
+$ docker push <registry>:5000/my-vm-img
+```