Not installing security updates is bad advice (#614)

This is obviously bad advice: security updates are important, and should be installed.

For more detailed explanation of why advice against this is bad, see https://pythonspeed.com/articles/security-updates-in-docker/

cheatsheets/Docker_Security_Cheat_Sheet.md

@@ -257,7 +257,6 @@ Many issues can be prevented by following some best practices when writing the D
 - Ensure the base image version is pinned
 - Ensure the OS packages versions are pinned
 - Avoid the use of `ADD` in favor of `COPY`
-- Avoid the use of `apt/apk upgrade`
 - Avoid curl bashing in `RUN` directives
 
 References: