Allow split configurations by default: import conf/*.toml by default.

[Romain-Geissler-1A]

Distro vendor might want to customize their containerd configuration and might prefer to split it in multiple files. Usually this is done with a /etc/tool.d/*.conf scheme (for example /etc/sysctl.d). This would allow distro to put any toml file in /etc/containerd/conf in Linux to customize the default configuration.

[k8s-ci-robot]

Hi @Romain-Geissler-1A. Thanks for your PR.

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[kzys]

Hmm, Linux distributions can have its own /etc/contaiainerd/config.toml. Do you know distros that need this change?

[Romain-Geissler-1A]

Distros no, but I know in my own company this is something we would like to have. Most software I know do accept a "software.conf.d" default directory where config file can be dropped and included automatically, and this allows companies/site specific config to be easily distributed via various way and be taken into account. The problem with a single config file is that usually this config file is distributed by the distro, and thus any form of customization becomes a maintenance burden as you have to constantely keep up with the distro update, and this because much more a problem when inside the same organisation you support N distro/distro version. With standalone separate config file which are auto-loaded the distribution and maintenance is easier.

[Romain-Geissler-1A]

One real world use case I have is configuring kind which right now doesn't default to using version 2 mirror configuration, via a separate file rather than via a kind containerd config patch which affects the global configuration. In our organization like in many others we shall access docker registry via internal mirrors rather than directly targetting the reigstries via internet.