compose: add build.extra_hosts to v3.9 schema

[thaJeztah]

fixes#1293

This is not currently used by the CLI, but can be used by docker compose to bring parity on this feature with the compose v2.4 schema.

relates to docker/compose#7323

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

@silvin-lubecki@ndeloof PTAL

[silvin-lubecki]

LGTM

[ndeloof]

Compose canonical schema should be https://github.com/compose-spec/compose-spec/blob/master/schema/config_schema_v3.9.json and such change should only take place after a public proposal to the compose-spec community.

Please note re-introducing v2 attributes has been discussed on compose-spec/compose-spec#12

[thaJeztah]

@ndeloof what I understood is that the current 3.9 spec was used as the transition to make compose-spec the canonical place because it was already worked on for the upcoming 20.03 release; if compose-spec is the canonical place, should this repository vendor the spec from that location? (also this means we have a release blocker for 20.03, pending #2503

[agners]

@thaJeztah@ndeloof this is utterly frustrating for outside contributors. I created a merge request months ago adding device_cgroup_rules (see #2206). However it got ignored/sidelined. With the compose-spec comming along, I did all my home work and helped making it part of the upcoming spec compose-spec/compose-spec#63.

And now, within 4 days other properties make it in directly into CLI's compose spec? So now compose-spec essentially forked??

[ndeloof]

https://github.com/compose-spec/compose-go is the exact same codebase as docker/cli for compose file loading and mdel (this is a git extract, including history). CLI should be updated to adopt compose-go and throw avoid code to diverge like this. This is more a question of finding the right timing vs CLI release schedule.

[agners]

@ndeloof@thaJeztah since we won't adopt compose-go in time for 20.03, could we then not just merge #2206? I would mean less divergence from compose-spec...

[ndeloof]

compose-spec is going to re-introduce all v2 attributes in the schema by approval of compose-spec/compose-spec#12
so #2206 still make sense, this won't introduce a divergence.

[burka]

For anyone searching like me: It's solved in Version 3.9 #2517 - you can add extra_hosts: in build: now.