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.
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?