output

debian

 label  | count 
--------+-------
 CZ0100 |     1
 CZ020A |     1
 CZ020B |     1
 CZ020C |     1
 CZ0311 |     1
 CZ0312 |     1
 CZ0313 |     1

alpine

 label  | count 
--------+-------
 CZ0100 |     1
 CZ0201 |     1
 CZ0202 |     1
 CZ0203 |     1
 CZ0204 |     1
 CZ0205 |     1
 CZ0206 |     1
 CZ0207 |     1
 CZ0208 |     1
 CZ0209 |     1
 CZ020A |     1
 CZ020B |     1
 CZ020C |     1
 CZ0311 |     1
 CZ0312 |     1
 CZ0313 |     1

plan

debian

                                          QUERY PLAN                                           
-----------------------------------------------------------------------------------------------
 GroupAggregate  (cost=203.28..204.16 rows=16 width=40)
   Output: cz__t_nuts.label, count(*)
   Group Key: cz__t_nuts.label
   ->  Merge Join  (cost=203.28..203.92 rows=16 width=32)
         Output: cz__t_nuts.label
         Merge Cond: (cz__t_nuts.label = en__t_nuts.label)
         ->  Foreign Scan on cz.t_nuts cz__t_nuts  (cost=101.48..101.84 rows=16 width=7)
               Output: cz__t_nuts.id, cz__t_nuts.label
               Remote SQL: SELECT label FROM public.t_nuts ORDER BY label ASC NULLS LAST
         ->  Sort  (cost=101.80..101.84 rows=16 width=7)
               Output: en__t_nuts.label
               Sort Key: en__t_nuts.label
               ->  Foreign Scan on en.t_nuts en__t_nuts  (cost=100.00..101.48 rows=16 width=7)
                     Output: en__t_nuts.label
                     Remote SQL: SELECT label FROM public.t_nuts

alpine

                                          QUERY PLAN                                           
-----------------------------------------------------------------------------------------------
 GroupAggregate  (cost=203.28..204.16 rows=16 width=40)
   Output: cz__t_nuts.label, count(*)
   Group Key: cz__t_nuts.label
   ->  Merge Join  (cost=203.28..203.92 rows=16 width=32)
         Output: cz__t_nuts.label
         Merge Cond: (cz__t_nuts.label = en__t_nuts.label)
         ->  Foreign Scan on cz.t_nuts cz__t_nuts  (cost=101.48..101.84 rows=16 width=7)
               Output: cz__t_nuts.id, cz__t_nuts.label
               Remote SQL: SELECT label FROM public.t_nuts ORDER BY label ASC NULLS LAST
         ->  Sort  (cost=101.80..101.84 rows=16 width=7)
               Output: en__t_nuts.label
               Sort Key: en__t_nuts.label
               ->  Foreign Scan on en.t_nuts en__t_nuts  (cost=100.00..101.48 rows=16 width=7)
                     Output: en__t_nuts.label
                     Remote SQL: SELECT label FROM public.t_nuts

version

debian

                                                     version                                                      
------------------------------------------------------------------------------------------------------------------
 PostgreSQL 12.4 (Debian 12.4-1.pgdg100+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 8.3.0-6) 8.3.0, 64-bit

alpine

                                        version                                        
---------------------------------------------------------------------------------------
 PostgreSQL 12.4 on x86_64-pc-linux-musl, compiled by gcc (Alpine 9.3.0) 9.3.0, 64-bit

full out files

fdw_collation_alp.out

fdw_collation_deb.out

version of glibc / musl libc

alpine

cat /etc/alpine-release 
3.12.0

ldd --version
musl libc (x86_64)
Version 1.1.24
Dynamic Program Loader
Usage: /lib/ld-musl-x86_64.so.1 [options] [--] pathname

debian / ubuntu

cat /etc/debian_version 
10.5

ldd --version
ldd (Debian GLIBC 2.28-10) 2.28
Copyright © 2018 Free Software Foundation, Inc.