Add `build.network` field to Compose file v3.x

[dnephin]

See docker/compose#4997

[dgageot]

The idea is to make sure the v3.4 of the schema supports (and ignores) build/network properties.
V3.4 of the schema is not yet merged in master. It's introduces in #358

[dnephin]

There's also a target field to add to the build section. IT would be good to add them both at the same time.

[dnephin]

#430