feat: enhance user account management during VM creation with automatic user creation and metadata preservation

README.md

@@ -351,13 +351,20 @@ The installed kernel and rootfs are templates. `create` copies the rootfs to
 the VM runtime directory before Firecracker opens it read-write, so VMs do not
 share a mutable root disk.
 
-During creation, UVM updates the private ext4 disk with a password for an
-existing guest account and enables OpenSSH password authentication. The
+During creation, UVM updates the private ext4 disk with a password for the
+requested guest account and enables OpenSSH password authentication. Missing
+custom usernames are created with an unused regular-user UID, a primary group,
+a private `/home/USERNAME` directory, and `/bin/sh` as their login shell. The
 defaults are username `root` and password `root`. A custom image must provide:
 
 - `sshd`
 - `/etc/passwd`, `/etc/shadow`, and `/etc/ssh/sshd_config`
-- The requested user account; UVM does not create missing users
+- `/etc/group` and `/bin/sh` when creating a new user
+
+Existing accounts retain their UID, groups, home directory, and shell. A new
+user reuses a same-named group if present; otherwise UVM creates one with an
+unused GID. `/etc/gshadow` is updated when present. The default `root` account
+must already exist in the image.
 
 The known public demo key bundled in Firecracker's default bionic image is
 removed from each private disk. UVM also replaces the conventional RSA, ECDSA,