Updated Polyverse reinstall commands in Dockerfile (#5173)

The problem: After this PR was merged (https://github.com/netdata/netdata/pull/5137), we noticed on our end that the default Polyverse install script had room for improvement to ensure there's reliability. To be more specific, we noticed that while we only expose x86_64 packages, we noticed a pull to an x86 (32-bit package) that was not expected. What this meant was, during our first update, we were completely removing existing mirrors/packages in the install script. The fix: Our install script now merely comments out the existing repos/mirrors. The install instructions then merely update/upgrade all packages in-place, this preferring our packages as a priority, and the final command uncomments all the other mirrors/repos that may exist. The outcome: Due to this step, ALL existing repos stay on the host after Polymorphic repositories are sbuscribed. Everything works as expected. However, the host will prefer Polymorphic repositories above others.