docs: Add some tips on getting a rootfs

#104 (review)
prompted me to try an experimentm it turns out conatiner runtimes
provide a super lightweight way to produce a rootfs.

Also provide a minimal example for mkosi.

This won't work without a user namespace so I guess we shouldn't merge
this until after #104 or something
simliar is in place.

README.md

@@ -179,6 +179,8 @@ aarch64
 
 For full configuration documentation, see [config.md](./docs/config.md).
 
+For tips on getting a rootfs, see [rotfs.md](./docs/rootfs.md).
+
 ## Usage in Github CI
 
 [vmtest-action](https://github.com/danobi/vmtest-action) is a convenient

docs/config.md

@@ -47,6 +47,8 @@ The following fields are supported:
     * UIDs from the host will be passed through directly; if you built your
       rootfs without privileges in the host, try running `vmtest` via
       `unshare -r`, so that QEMU (and hence the guest) sees UID 0.
+    * For tips on getting a rootfs, see [rotfs.md](rootfs.md).
+
 * `arch` (string)
     * Default: the architecture vmtest was built for.
     * Under which machine architecture to run the kernel.

docs/rootfs.md

@@ -0,0 +1,51 @@
+# Getting a rootfs
+
+There are many ways to produce a directory to pass to the `rootfs` config field,
+here are a couple of potential solutions.
+
+## From a container image
+
+OCI images can be turned into tarballs which can be extracted into a rootfs. For
+example:
+
+```sh
+❯❯ mkdir $rootfs_dir && cd $rootfs_dir
+❯❯ cat > Containerfile
+FROM docker.io/library/debian
+RUN apt update
+RUN apt install -y qemu-guest-agent
+
+❯❯ podman build -t deb-qga  # Docker would work exactly the same
+❯❯ podman export -o deb.tar $(podman create deb-qga)
+❯❯ tar xf deb.tar
+❯❯ rm Containerfile deb.tar
+```
+
+## Using mkosi
+
+[`mkosi`](https://github.com/systemd/mkosi) is a more advanced tool for building
+OS images, as well as just producing a rootfs it can build full disk images with
+a bootloader, plus many other features. You'll need to refer to the full
+documentation to really understand `mkosi`, but here's a minimal example. This
+will only work if you host system has `apt`, otherwise you'll need to adapt it
+for your host distro or run it in a container.
+
+`mkosi.conf`:
+
+```ini
+[Output]
+Format=directory
+
+[Distribution]
+Distribution=debian
+Release=testing
+
+[Content]
+Packages=
+        mount
+        qemu-guest-agent
+```
+
+Then from the directory containing that file, run `mkosi -f`. This should
+produce a directory named `image` that you can use for your `rootfs` config
+field.