Please, what does:
select * from pg_stat_replication
show on the primary?
It shows 0 rows.
Here is the pg_xlog folder from standby:
root@cba9f6ba3f6e:/# cat ${PGDATA}/pg_xlog/
00000001000000000000000C archive_status
And here it is from the primary:
root@ad4a83bdcf9e:/# cat ${PGDATA}/pg_xlog/
000000010000000000000005
000000010000000000000006
000000010000000000000007
000000010000000000000008
000000010000000000000009
00000001000000000000000A
00000001000000000000000B
00000001000000000000000C
00000001000000000000000C.00000028.backup
00000001000000000000000D
00000001000000000000000E
archive_status