Fix panic in buildx debug when solving fails

Closes #3822

Signed-off-by: Mateusz Gozdek <mgozdek@microsoft.com>

build/result.go

@@ -220,16 +220,24 @@ func containerConfigFromError(solveErr *errdefs.SolveError, cfg *InvokeConfig) (
 	if err != nil {
 		return nil, err
 	}
+	// When a solve fails before its inputs/mounts have been resolved (for
+	// example, when computing the cache key for an exec op fails because an
+	// input source cannot be found), MountIDs/InputIDs may be empty or shorter
+	// than the declared mount list. Guard against that so callers (such as the
+	// debug adapter) get a clean error instead of an index-out-of-range panic.
+	ids := solveErr.MountIDs
+	if cfg.Initial {
+		ids = solveErr.InputIDs
+	}
+	if len(ids) < len(exec.Mounts) {
+		return nil, errors.Errorf("cannot start debug container: failing step has no mount results available (got %d of %d)", len(ids), len(exec.Mounts))
+	}
 	var mounts []gateway.Mount
 	for i, mnt := range exec.Mounts {
-		rid := solveErr.MountIDs[i]
-		if cfg.Initial {
-			rid = solveErr.InputIDs[i]
-		}
 		mounts = append(mounts, gateway.Mount{
 			Selector:  mnt.Selector,
 			Dest:      mnt.Dest,
-			ResultID:  rid,
+			ResultID:  ids[i],
 			Readonly:  mnt.Readonly,
 			MountType: mnt.MountType,
 			CacheOpt:  mnt.CacheOpt,