The container is built from a trivial Dockerfile:
FROM php:7.2.19-fpm
RUN docker-php-ext-configure opcache --enable-opcache \
&& docker-php-ext-install opcache
I did not change php.ini, so all the settings are default.
What's unexpected is that now phpinfo shows (the excerpt):
Opcode Caching Up and Running
Optimization Disabled
SHM Cache Enabled
File Cache Disabled
opcache.enable On On
opcache.optimization_level 0 0x7FFFBFFF
The question: what else do I need to do to have optimisations enabled?
Well, my apologies, I over-simplified the question to a degree when it's impossible to answer it. I'm really sorry for that.
The real reason it's not enabled is because xdebug is installed and enabled as well.