How to flush WAL files?

I'm trying to force WAL files to be flushed on to the disk so that I can copy them somewhere else. I believe [CHECKPOINT](https://www.postgresql.org/docs/current/sql-checkpoint.html) is the command I'm interested in but it doesn't seem to work. I have a docker container of Postgres running and I use adminer to add new records to a table I created. I tried to flush the WAL files manually:

$ docker exec db psql pgtest -U postgres -c "CHECKPOINT;"
CHECKPOINT

I believe that should flush the data and I would see a new WAL file created, however there are no new files - I only see the old WAL files. When I restart the container:

$ docker restart db
db

Then I see a new WAL file. This is what my postgresql.conf looks like:

#------------------------------------------------------------------------------
# WRITE AHEAD LOG
#------------------------------------------------------------------------------

# - Settings -

wal_level = logical                   # minimal, replica, or logical
                                      # (change requires restart)
fsync = on                            # flush data to disk for crash safety
                                      # (turning this off can cause
                                      # unrecoverable data corruption)
synchronous_commit = local            # synchronization level;
                                      # off, local, remote_write, remote_apply, or on
wal_sync_method = fsync               # the default is the first option
                                      # supported by the operating system:
                                      #   open_datasync
                                      #   fdatasync (default on Linux)
                                      #   fsync
                                      #   fsync_writethrough
                                      #   open_sync
#full_page_writes = on                # recover from partial page writes
#wal_compression = off                # enable compression of full-page writes
#wal_log_hints = off                  # also do full page writes of non-critical updates
                                      # (change requires restart)
#wal_buffers = -1                     # min 32kB, -1 sets based on shared_buffers
                                      # (change requires restart)
#wal_writer_delay = 200ms             # 1-10000 milliseconds
wal_writer_flush_after = 1MB          # measured in pages, 0 disables

#commit_delay = 0                     # range 0-100000, in microseconds
#commit_siblings = 5                  # range 1-1000

# - Checkpoints -

#checkpoint_timeout = 1min            # range 30s-1d
#max_wal_size = 1GB
#min_wal_size = 4MB
#checkpoint_completion_target = 0.5   # checkpoint target duration, 0.0 - 1.0
#checkpoint_flush_after = 256kB       # measured in pages, 0 disables
#checkpoint_warning = 30s             # 0 disables

# - Archiving -

archive_mode = on                     # enables archiving; off, on, or always
                                      # (change requires restart)
archive_command = 'test ! -f /var/lib/postgresql/data/wal/%f && cp %p /var/lib/postgresql/data/wal/%f'
                                      # placeholders: %p = path of file to archive
                                      #               %f = file name only
                                      # e.g. 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'
#archive_timeout = 0                  # force a logfile segment switch after this
                                      # number of seconds; 0 disables