Similar to #14318 and #14170. Some Config/HostConfig options suffer from a regression where previously (<1.7.0) it was possible to just pass a string in json and be adapted to be a strings slice (old engine GetenvList) while now it isn't possible anymore cause we are directly decoding in []string

Dns and DnsSearch are suffering the same issue (compose moves a single string to a list if there's only one element https://github.com/docker/compose/blob/77c939b256dbed699fdc4643114bea543208a660/compose/service.py#L570 so this could have been missed)
I'm fixing Dns and DnsSearch though, and I still haven't looked at all the options

/cc @aanand @calavera @LK4D4