0e3e1012ac
As for top-level key, any 3rd-level key which starts with `x-` will be
ignored by compose. This allows for users to:
* include additional metadata in their compose files
* create YAML anchor objects that can be re-used in other parts of the config
This matches a similar feature in the swagger spec definition:
https://swagger.io/specification/#specificationExtensions
This means a composefile like the following is valid
```
verison: "3.7"
services:
foo:
image: foo/bar
x-foo: bar
network:
bar:
x-bar: baz
```
It concerns services, volumes, networks, configs and secrets.
Signed-off-by: Vincent Demeester <vincent@sbr.pm>
Upstream-commit: 6bcfa8806c
Component: cli