WORKSPACE: Update io_bazel_rules_docker to latest v0.20.0
WORKSPACE: Update io_bazel_rules_docker to latest v0.20.0

This was released on 2021-10-11. It seems like a reasonable idea to
track the latest upstream release, but I am not aware of any specific
reason or issue to require this update.

https://github.com/bazelbuild/rules_docker/releases/tag/v0.20.0

GoogleContainerTools/distroless,GoogleContainerTools/distroless,GoogleContainerTools/distroless