gh-159452: Run the regen-configure container as the invoking user (#159454)

`Tools/build/regen-configure.sh` ran the Autoconf container as root, so
with a rootful Docker daemon the regenerated files and `autom4te.cache`
ended up owned by root. The stale cache broke subsequent runs and
couldn't be removed without sudo.

Run the container as the invoking user instead: `--userns=keep-id` for
Podman, `-u UID:GID` for rootful Docker. Rootless runtimes already map
container root to the invoking user and need nothing. Also remove
`autom4te.cache` before and after the run, using the same image, so an
existing root-owned cache gets cleaned up too.

Author: sendaoYan

Misc/NEWS.d/next/Build/2026-10-15-17-19-41.gh-issue-159452.oJ1saO.rst

@@ -0,0 +1,2 @@
+``Tools/build/regen-configure.sh`` no longer leaves
+root-owned files in the source tree.

Tools/build/regen-configure.sh

@@ -26,4 +26,27 @@ if command -v selinuxenabled >/dev/null && selinuxenabled; then
     PATH_OPT=":Z"
 fi
 
-"$RUNTIME" run --rm -v "$abs_srcdir:$WORK_DIR$PATH_OPT" "$IMAGE"
+# Run the container as the invoking user so that generated files are owned by
+# that user and not by root. Rootless runtimes already map root to the invoking
+# user, so a user mapping is only needed for rootful Docker.
+USER_OPT=""
+case "$RUNTIME" in
+    podman)
+        USER_OPT="--userns=keep-id"
+        ;;
+    docker)
+        if ! docker info --format '{{.SecurityOptions}}' | grep -q rootless; then
+            USER_OPT="-u $(id -u):$(id -g)"
+        fi
+        ;;
+esac
+
+purge_autom4te_cache() {
+    [ -d "$abs_srcdir/autom4te.cache" ] || return 0
+    "$RUNTIME" run --rm -v "$abs_srcdir:$WORK_DIR$PATH_OPT" \
+        "$IMAGE" sh -c "rm -rf $WORK_DIR/autom4te.cache"
+}
+
+purge_autom4te_cache
+trap 'purge_autom4te_cache || true' EXIT
+"$RUNTIME" run --rm $USER_OPT -v "$abs_srcdir:$WORK_DIR$PATH_OPT" "$IMAGE"