That looks OK to me. I think it should drop all metrics with name "container_processes".
Can you show a wider context of the entire scrape job config? metric_relabel_configs is configured under a specific scrape job. and only applies to metrics collected by that scrape job.
There are simple things to check, like did you send prometheus a HUP or reload signal after changing the config? And if you check prometheus output (typically "journalctl -eu prometheus") did it say the reload was successful, with no syntax errors in the new config?
Are you deploying prometheus via some higher-level wrapper, like a helm chart? If so, the issue is likely somewhere around that.