fix(entrypoint): unlink planted symlinks as the customer, then fail closed

Root rm -f in drop_symlink could be redirected by a parent-directory swap to
delete arbitrary files as root. drop_symlink and the logs/ symlink sweep now
unlink AS THE CUSTOMER (can never delete anything the customer couldn't), and
drop_symlink still refuses to start if the path is a symlink afterwards; the
pre-daemon logs/ gate re-checks and fails closed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/entrypoint.sh

@@ -37,12 +37,14 @@ die() {
   echo "entrypoint: FATAL: $* - refusing to start" >&2
   exit 1
 }
-# Unlink as root: rm on the final component never touches the link target and
-# the customer cannot deny it. Still a symlink afterwards -> refuse to start.
+# Unlink AS THE CUSTOMER: rm on the final component never touches the link
+# target, and if a parent dir is swapped for a symlink mid-way the customer can
+# only delete what they already could. Still a symlink afterwards -> refuse to
+# start (fail closed).
 drop_symlink() {
   if [ -L "$1" ]; then
     echo "entrypoint: SECURITY: $1 was a symlink to '$(readlink "$1")' - removing" >&2
-    rm -f -- "$1"
+    as_user rm -f -- "$1"
     [ -L "$1" ] && die "could not remove symlink $1"
   fi
   return 0
@@ -55,9 +57,11 @@ chown -h $user:$user /home/$user
 # alone, and every root-side operation on it below runs as the customer.
 drop_symlink /home/$user/logs
 if [ -d /home/$user/logs ]; then
-  # nginx opens its logs here as root. find without -L never follows symlinks;
-  # -delete unlinks the link itself. Re-checked fail-closed before nginx starts.
-  find /home/$user/logs -xdev -type l \
+  # nginx opens its logs here as root. As the customer: find without -L never
+  # follows symlinks, -delete unlinks the link itself, and a raced parent swap
+  # can only reach what the customer could already delete. Re-checked
+  # fail-closed immediately before nginx starts.
+  as_user find /home/$user/logs -xdev -type l \
     -printf 'entrypoint: SECURITY: %p was a symlink to %l - removed\n' -delete >&2 || true
 fi