clean up & simplify structure w/ separate build workflows

Independent NGINX & PHP-FPM build workflows & directory structure for
cleaner and simpler workflow logic.

shared/php-fpm/entrypoint.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+set -euo pipefail
+
+: ${CHOWN_ON_START:=}
+if [ "${CHOWN_ON_START}" = "1" ] || [ "${CHOWN_ON_START}" = "true" ]; then
+  echo "[entrypoint] CHOWN_ON_START enabled — fixing ownership of /var/www/html"
+  if [ -d /var/www/html ]; then
+    chown -R 1000:1000 /var/www/html || true
+  fi
+fi
+
+exec "$@"