Add isolation parameter to v3 compose file schema (fixes #5069).

[Techwolfy]

Fix for isolation parameter errors in v3+ compose files.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:Techwolfy/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[shin-]

As I explained in #5069 (comment), we can not make this change unless it is an available option for Swarm services. Moreover, the v3 spec should be modified on the docker/cli repo first then reproduced here - not the other way around.