In my main PHP service, I stumbled on a permission error while my app was trying to create a directory in /app/var/cache, causing a PHP exception when accessing my app through HTTP.

Full error message is Failed to create "/app/var/cache/prod/asset_mapper": mkdir(): Permission denied

Note that I didn't have this issue when fiddling with my app in dev (that can be checked in the Staging environment).

I managed to workaround the issue by pre-compiling the assets during the build step, but I'm afraid the same issue would occur whenever a file is generated by my app.