feat: add --hostname flag to customize the vm hostname

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

docs/content/reference/d2vm_build.md

@@ -17,6 +17,7 @@ d2vm build [context directory] [flags]
   -f, --file string                Name of the Dockerfile
       --force                      Override output qcow2 image
   -h, --help                       help for build
+      --hostname string            Hostname to set in the generated image (default "localhost")
       --keep-cache                 Keep the images after the build
       --luks-password string       Password to use for the LUKS encrypted root partition. If not set, the root partition will not be encrypted
       --network-manager string     Network manager to use for the image: none, netplan, ifupdown