xtask: Prefer local bootc image for bcvk dev VM creation

When iterating with the bcvk dev VM, developers often have a locally
built composefs image (localhost/bootc). Automatically using that if it
exists avoids having to explicitly set BOOTC_BASE_IMAGE on every
invocation, and ensures the VM uses the correct bootloader (e.g.
systemd-boot) rather than the upstream base's grub.

Assisted-by: OpenCode (Claude Sonnet 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/xtask/src/sysext.rs

@@ -265,7 +265,20 @@ fn create_vm(sh: &Shell) -> Result<()> {
 
     let base_img = std::env::var("BOOTC_BASE_IMAGE")
         .or_else(|_| std::env::var("BOOTC_base"))
-        .unwrap_or_else(|_| "quay.io/centos-bootc/centos-bootc:stream10".to_string());
+        .unwrap_or_else(|_| {
+            // Prefer localhost/bootc if it exists (local composefs build), otherwise
+            // fall back to the upstream stream10 base.
+            let local_exists = Command::new("podman")
+                .args(["image", "exists", "localhost/bootc"])
+                .status()
+                .map(|s| s.success())
+                .unwrap_or(false);
+            if local_exists {
+                "localhost/bootc".to_string()
+            } else {
+                "quay.io/centos-bootc/centos-bootc:stream10".to_string()
+            }
+        });
     let bind_mount = format!("{}:{}", sysext_path, VM_SYSEXT_MNT);
 
     let bcvk_opts = BcvkInstallOpts::from_env();