Respect system/user settings in registries.conf[.d]

[andremarianiello]

Right now I am working in an environment where docker.io images are pulled through a read-through cache to avoid hitting docker.io rate limits. The mirroring and caching is configured on systems using registries.conf/registries.conf.d. However, I hit the docker rate limits when using osbuild-composer. I think I tracked the root cause down to this bit of code.

		sysCtx: &types.SystemContext{
			RegistriesDirPath:        "",
			SystemRegistriesConfPath: "",
			BigFilesTemporaryDir:     "/var/tmp",

			OSChoice: "linux",

			AuthFilePath: GetDefaultAuthFile(),
		},

SystemRegistriesConfPath and SystemRegistriesConfigDir are not set. Setting these the same way podman does would solve my problem, and probably any more that would result from config drift.