Provide image-defaults for `CF_OVERRIDES_EXCLUSIONS`

[itzg]

Provide image-defaults for CF_OVERRIDES_EXCLUSIONS similar to existing support for CF_EXCLUDE_MODS.

Discussed in Discord

Could add a new section to https://github.com/itzg/docker-minecraft-server/blob/master/files/cf-exclude-include.json that gets processed by mc-image-helper.