php-fpm: improve dockerfile, switch to per-lane www.conf

Shared www.conf isn't viable due to differing supported options between
PHP-FPM versions. Additionally, the Dockerfile multi-stage build wasn't
useful as it copied build artifacts. Switched to single-stage with build
artifact pruning.

shared/php-fpm/www.conf

@@ -1,31 +0,0 @@
-; Shared php-fpm pool configuration for containers
-
-[www]
-listen = 0.0.0.0:9000
-
-user = app
-group = app
-
-listen.owner = app
-listen.group = app
-listen.mode = 0660
-
-pm = dynamic
-pm.max_children = 10
-pm.start_servers = 2
-pm.min_spare_servers = 1
-pm.max_spare_servers = 3
-pm.max_requests = 500
-
-request_terminate_timeout = 300s
-request_slowlog_timeout = 5s
-slowlog = /var/log/php/www-slow.log
-
-catch_workers_output = yes
-access.log = /var/log/php/www-access.log
-
-clear_env = no
-
-emergency_restart_threshold = 10
-emergency_restart_interval = 1m
-process_control_timeout = 10s