- Add a new docker image "rbe_ubuntu2004" that is built in a way that's
analogous to how our other testing docker images are built (this gives
us control over what exactly is contained in the docker image and
ability to fine-tune our RBE configuration)
- Switch RBE on linux to the new image (which gives us ubuntu20.04-based
builds)
For some reason, RBE seems to have trouble pulling the docker image from
Google Artifact Registry (GAR), which is where our public testing images
normally live, so for now, I used a workaround and I upload a copy of
the rbe_ubuntu2004 docker image to GCR as well, and that makes RBE works
just fine (see comment in the `renerate_linux_rbe_configs.sh` script).
More followup items (config cleanup, getting local sanitizer builds
working etc.) are in go/grpc-rbe-tech-debt-2023
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