Set sensible defaults for PHP execution time and POST/upload sizes

[kawaii]

The following snippet should be added to the Dockerfile to override the PHP defaults for uploaded files and script execution time.

RUN { \
                echo 'file_uploads=On'; \
                echo 'max_execution_time=20'; \
                echo 'post_max_size=10M'; \
                echo 'upload_max_filesize=10M'; \
                echo 'memory_limit=256M'; \
        } > /usr/local/etc/php/conf.d/mybb-recommended.ini

Could I get some feedback from @mybb/developers and @mybb/sqa with regards to some numbers that we all agree on please?