php-fpm -t コマンドで探す

$ docker run php:8.3-fpm php-fpm -t
[01-Sep-2024 19:57:29] NOTICE: configuration file /usr/local/etc/php-fpm.conf test is successful

$ docker run php:8.3-fpm ls /usr/local/etc/php-fpm.conf
/usr/local/etc/php-fpm.conf

見つかりました！

補足

$ docker run php:8.3-fpm php-fpm -h
Usage: php [-n] [-e] [-h] [-i] [-m] [-v] [-t] [-p <prefix>] [-g <pid>] [-c <file>] [-d foo[=bar]] [-y <file>] [-D] [-F [-O]]
  -c <path>|<file> Look for php.ini file in this directory
  -n               No php.ini file will be used
  -d foo[=bar]     Define INI entry foo with value 'bar'
  -e               Generate extended information for debugger/profiler
  -h               This help
  -i               PHP information
  -m               Show compiled in modules
  -v               Version number
  -p, --prefix <dir>
                   Specify alternative prefix path to FastCGI process manager (default: /usr/local).
  -g, --pid <file>
                   Specify the PID file location.
  -y, --fpm-config <file>
                   Specify alternative path to FastCGI process manager config file.
  -t, --test       Test FPM configuration and exit
  -D, --daemonize  force to run in background, and ignore daemonize option from config file
  -F, --nodaemonize
                   force to stay in foreground, and ignore daemonize option from config file
  -O, --force-stderr
                   force output to stderr in nodaemonize even if stderr is not a TTY
  -R, --allow-to-run-as-root
                   Allow pool to run as root (disabled by default)