Why would Postgres 11 suddenly ignore the recovery.conf file?

Hi,
I just stopped and restarted a Postgres 11 docker container which so far 
has happily streamed the WAL from its primary but after the restart it 
now seems to completely ignore the recovery.conf file and just outputs this:

  2021-10-04 10:14:19.103 UTC [1] LOG:  listening on IPv4 address 
"0.0.0.0", port 5432
2021-10-04 10:14:19.103 UTC [1] LOG:  listening on IPv6 address "::", 
port 5432
2021-10-04 10:14:19.123 UTC [1] LOG:  listening on Unix socket 
"/var/run/postgresql/.s.PGSQL.5432"
2021-10-04 10:14:19.304 UTC [21] LOG:  database system was shut down in 
recovery at 2021-10-04 10:14:11 UTC
2021-10-04 10:14:19.305 UTC [21] LOG:  entering standby mode
2021-10-04 10:14:19.313 UTC [21] LOG:  could not signal for checkpoint: 
checkpointer is not running
2021-10-04 10:14:19.338 UTC [21] LOG:  redo starts at 2F01/B9044C30
2021-10-04 10:15:35.889 UTC [21] LOG:  consistent recovery state reached 
at 2F02/A2E1A108
2021-10-04 10:15:35.893 UTC [1] LOG:  database system is ready to accept 
read only connections

Notice that there is apparently no attempt being made to connect to the 
primary any more. The recovery.conf file is still present and correct so 
I don't understand why Postgres seems to ignore it completely?

Regards,
   Dennis