Build args in dockerfiles should not use realistic values

[jameslamb]

Description

Dockerfiles in this repo currently set realistic values as defaults for build arguments

https://github.com/rapidsai/dask-build-environment/blob/e121aa88f172c7cfe83c2774125fd9d33a145dc6/dask/Dockerfile#L1-L3

But never intentionally use those defaults... it's always expected that those build arguments will be fulfilled by values passed through to docker build:

https://github.com/rapidsai/dask-build-environment/blob/e121aa88f172c7cfe83c2774125fd9d33a145dc6/ci/gpuci/run.sh#L64

Those realistic values should be replaced with the literal string unset or similar.

Benefits of this work

Reduces the risk of silently building and tagging an image that's using the wrong configuration (because of mistakes in passing through build args).

Acceptance Criteria

• all configuration for build arguments is driver from outside of Dockerfiles

Approach

This: https://github.com/rapidsai/docker/pull/689#discussion_r1684918803

Notes

Created this issue based on https://github.com/rapidsai/dask-build-environment/pull/97#discussion_r1729432517