v2.2 service init option not supported in v3

[LukeusMaximus]

PR #4716 added init to docker-compose services in v2.2 compose files, but not v3. Please can v3 be made to support the init option as well.

[shin-]

v3 format requests should be created on moby/moby

[ionelmc]

@shin- Umm wat? What's special about the v3? I don't get it. Why wasn't v3 support added in #4716?

[shin-]

@ionelmc v3 is the format for Swarm stacks. It is (partially) compatible with docker-compose, but updates to it are driven at the engine level, not here.