That field is automaticaly populated with any `x-*` field in the yaml.
And marshalling the compose config struct put them back into place.
This make it possible to get those extra fields without re-inventing
the wheel (i.e. reimplementing 80% of the `cli/compose/*` packages.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
Upstream-commit: 80c26f618e
Component: cli