If you face this error when you start Symfony project with docker (my Symfony version 5.1). Or errors like these:
Uncaught Exception: Failed to write file "/var/www/html/mysite.com.local/var/cache/dev/App_KernelDevDebugContainer.xml"" while reading upstream
Uncaught Warning: file_put_contents(/var/www/html/mysite.com.local/var/cache/dev/App_KernelDevDebugContainerDeprecations.log): failed to open stream: Permission denied" while reading upstream
Fix below helped me.
In Dockerfile for nginx container add line:
RUN usermod -u 1000 www-data
In Dockerfile for php-fpm container add line:
RUN usermod -u 1000 www-data
Then remove everything in directories "/var/cache", "/var/log" and rebuild docker's containers.