Need a plan for Docker images to update helix scripts

[michellemcdaniel]

Migrated from https://github.com/dotnet/core-eng/issues/15465:

The docker images just grab whatever version of the helix scripts happens to exist when they are built. This means they are horribly out of date, leading to issues like https://github.com/dotnet/core-eng/issues/14718. Basically any bug is enshrined in them more or less forever.

In addition, this creates a huge compatibility matrix we can't afford to maintain, where every version of helix we've ever published needs to interoperate with every future version of helix. This is incredibly costly and difficult to test for (since you'd need to run N^2 tests, testing every possible combination of versions).

We need a plan to make sure the scripts inside the docker containers are updatable.

Prior to the whl, it wasn't necessary, since we just copied the scripts into the docker contain ouselves, but the whl file has native file dependencies that get restored inside, causing that to no longer be an option.