Run container as unprivileged www-data user and simplify entrypoint

- Remove root-only FPM user configuration from entrypoint.sh (moved to Dockerfile build time)
- Remove Redis session configuration from entrypoint.sh (handled by Symfony services)
- Remove cache directory permission adjustments (unnecessary with proper ownership)
- Set file ownership to www-data:www-data at build time using COPY --chown
- Create required directories and set ownership in single RUN layer
- Switch to www-data USER before CMD
- Remove PHP_FPM_USER, PHP_FPM_GROUP, SESSION_LIFETIME environment variables
- Add docker/ directory to .dockerignore

.gitignore

@@ -1,4 +1,4 @@
-
+docker/
 ###> symfony/framework-bundle ###
 /.env.local
 /.env.local.php