fix: add missing openldap-overlay-syncprov package

The syncprov.so module was being loaded in init-config.sh but the
required Alpine package was not installed, causing slapadd to fail
with "file not found" error.

Dockerfile

@@ -12,6 +12,7 @@ RUN apk add --no-cache \
     openldap-overlay-refint \
     openldap-overlay-unique \
     openldap-overlay-ppolicy \
+    openldap-overlay-syncprov \
     openssl \
     argon2 \
     && mkdir -p /var/lib/openldap/openldap-data \