* correct dockerfile instructions for boost php settings

https://github.com/docker-library/docs/blob/master/php/README.md#how-to-install-more-php-extensions

wp/Dockerfile

@@ -1,8 +1,7 @@
 FROM wordpress:5.7.2
-RUN cd /usr/local/etc/php
-RUN cp php.ini-production php.ini
-RUN sed -i 's/upload_max_filesize = 2M/upload_max_filesize = 256M/' php.ini
-RUN sed -i 's/max_input_time = 60/max_input_time = 300/' php.ini
-RUN sed -i 's/memory_limit = 128M/memory_limit = 256M/' php.ini
-RUN sed -i 's/max_execution_time = 30/max_execution_time = 300/' php.ini
-RUN sed -i 's/post_max_size = 8M/post_max_size = 512M/' php.ini
+RUN mv "$PHP_INI_DIR/php.ini-production" "$PHP_INI_DIR/php.ini"
+RUN sed -i 's/upload_max_filesize = 2M/upload_max_filesize = 256M/' "$PHP_INI_DIR/php.ini"
+RUN sed -i 's/max_input_time = 60/max_input_time = 300/' "$PHP_INI_DIR/php.ini"
+RUN sed -i 's/memory_limit = 128M/memory_limit = 256M/' "$PHP_INI_DIR/php.ini"
+RUN sed -i 's/max_execution_time = 30/max_execution_time = 300/' "$PHP_INI_DIR/php.ini"
+RUN sed -i 's/post_max_size = 8M/post_max_size = 512M/' "$PHP_INI_DIR/php.ini"