cli/command/formatter: add CreatedAt to volume list output

[UditDewan]

• What I did

Added a CreatedAt placeholder to docker volume ls --format, which fixes #3871.

Containers, images and networks all expose CreatedAt in their list output, but volumes did not, even though the API has returned a CreatedAt field for volumes since API v1.42. As a result, the creation time was not reachable from docker volume ls --format '{{.CreatedAt}}' or docker volume ls --format json, and consumers had to fall back to docker volume inspect per volume.

• How I did it

Added a CreatedAt() method to volumeContext and registered the CREATED AT header, mirroring the other list formatters.

The API returns the value as an RFC3339 string, so it's parsed and rendered with time.Time.String(), matching what docker ps, docker images and docker network ls already emit for {{.CreatedAt}}. Two edge cases are handled explicitly:

• the field is omitempty in the API, so an empty string is returned when the daemon doesn't report a creation time (rather than a zero time)
• an unparsable value is passed through as-is rather than swallowed

The default table format is unchanged; this only adds a placeholder.

• How to verify it

$ docker volume create foo
$ docker volume ls --format json
{"Availability":"N/A","CreatedAt":"2026-08-02 21:04:11 +0000 UTC","Driver":"local","Group":"N/A","Labels":"","Links":"N/A","Mountpoint":"/var/lib/docker/volumes/foo/_data","Name":"foo","Scope":"local","Size":"N/A","Status":"N/A"}

$ docker volume ls --format 'table {{.Name}}\t{{.CreatedAt}}'
VOLUME NAME   CREATED AT
foo           2026-08-02 21:04:11 +0000 UTC

Unit tests were extended to cover the populated, empty and unparsable cases:

$ go test ./cli/command/formatter/ -run Volume -v

• Description for the changelog

Add a CreatedAt placeholder to the docker volume ls --format output.

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