That approach will mitigate your machine getting compromised (which is good) but it won't fix your production machines getting compromised if the gem or package gets deployed. That is usually a much worse outcome.

And even in isolated environments I find myself running code outside of the container for testing. Usually a quick script to test some package's functionality or opening a REPL to run something or running a code-generator (manage.py, artisan, etc). That's all it takes for the malware to break out of the isolation and attack your machine.