Repmgr : It automatically promotes to new master but other standby stopped
I've posted this response on GitHub:
At this point we haven't made any particular provision for repmgr to run in Docker, so it's possible there may be issues of one kind or another.

I also tried adding this
# service_start_command='pg_ctl -D ${PGDATA} start'
# service_stop_command='pg_ctl -D ${PGDATA} stop -m fast'
# service_reload_command='pg_ctl -D ${PGDATA} reload'
#service_restart_command='pg_ctl -D ${PGDATA} restart -m fast'

but same result.

Did you try adding these items without the leading #? I.e.
service_start_command='pg_ctl -D ${PGDATA} start'
service_stop_command='pg_ctl -D ${PGDATA} stop -m fast'
service_reload_command='pg_ctl -D ${PGDATA} reload'
service_restart_command='pg_ctl -D ${PGDATA} restart -m fast'

By default, when restarting a node for a standby follow operation, repmgr will stop then start the server using pg_ctl, as pg_ctl restart has proven to be problematic in some environments. However the opposite might be the case here. Either way we strongly recommend using the OS level service commands where available to avoid issues like this (not sure if those would be available here).