This updates the GPU image to run on the same Ubuntu version as the regular (non-GPU) image. This implicitly updates cmake etc for compatibility with newer versions of downstream libraries, e.g. Horovod.
Instead of installing dependencies in each Buildkite job, let's move this to the Dockerfile instead.
This will update GPU tests to always use Python 3.7.
* Revert "Revert "[Build] include minimal debug info in C++ build; upgrade clang-format to 12 (#18840)" (#18886)"
This reverts commit f851a072f3.
* use gcc 8
