file_put_contents(/root/.composer/cache/repo/https---flex.symfony.com/): failed to open stream: Is a directory error
This was a bug on Flex version <1.13.4
In latest version (as I write 1.13.4) the issue is solved.
The problem was:
Writing to the cache with an empty key will fail with "failed to openstream: Is a directory", so do not try to do that.
We noticed this when cloudflare - or the backend service - responded with a "last-modified" header for our CI servers (AWS) but not for our local system. This triggered the condition to become true and it tries to write a cache file without a filename.
That problem was solved with commit d81196c3f3b5
Edit: as posted by @Tmb a workaround is to use: composer install --no-cache ...