doc: build command

Introduce the `build` command.

Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

doc/01-usage.md

@@ -96,6 +96,22 @@ $ image-builder list-images --filter type:qcow2 --filter distro:fedora-41
 # ... list ...
 ```
 
+## `image-builder build`
+
+The `build` command builds images of a given [image type](./10-faq.md#image-types), for example:
+
+```console
+$ sudo image-builder build minimal-raw
+# ... progress ...
+```
+
+By default the `build` command uses the same distribution and version as the host system, you can pass another distribution and version with the `--distro` argument:
+
+```console
+$ sudo image-builder build --distro fedora-43 minimal-raw
+# ... progress ...
+```
+
 # Blueprints
 
 Images can be customized with [blueprints](https://osbuild.org/docs/user-guide/blueprint-reference). For example we could build the `qcow2` we built above with some customizations applied.

doc/10-faq.md

@@ -15,3 +15,9 @@ The above can be quite confusing, hence we've created `image-builder`. It allows
 ## Why does `image-builder` need `root` permissions?
 
 For image types where we need to work with filesystems we need root. Mounting and working with filesystems is not namespaced in the Linux kernel and mounting filesystems is generally considered to be "running untrusted code in the kernel" hence it requires root permissions.
+
+## Built-in distributions
+
+- Fedora
+- CentOS
+- RHEL