Support for this environment variable was removed in docker 23.0 in
1240f8b41d
From that patch:
> All regular, non-EOL Linux distros now come with more recent kernels
> out of the box. There may still be users trying to run on kernel 3.10
> or older (some embedded systems, e.g.), but those should be a rare
> exception, which we don't have to take into account.
>
> This patch removes the kernel version check on Linux, and the corresponding
> DOCKER_NOWARN_KERNEL_VERSION environment that was there to skip this
> check.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
