Idea: add opcache support

[jokesterfr]

Example:

# RUN docker-php-ext-enable opcache
# RUN echo '\
#   opcache.interned_strings_buffer=16\n\
#   opcache.load_comments=Off\n\
#   opcache.max_accelerated_files=16000\n\
#   opcache.save_comments=Off\n\
#   ' >> /usr/local/etc/php/conf.d/docker-php-ext-opcache.ini

Would be an interesting feature to add, but like the Xdebug mode: with a default opt-out, and only a OPCACHE_SUPPORT=true env would enable it.