Fix failure in `docker ps --format` when `.Label` has args
Fix failure in `docker ps --format` when `.Label` has args

This fix tries to fix the issue in 30279 where  `docker ps --format`
fails if `.Label` has args. For example:
```
docker ps --format '{{.ID}}\t{{.Names}}\t{{.Label "some.label"}}'
```

The reason for the failure is that during the preprocessing phase
to detect the existance of `.Size`, the `listOptionsProcessor`
does not has a method of `Label(name string) string`.

This results in the failure of
```
template: :1:24: executing "" at <.Label>: Label is not a method but has arguments
```

This fix fixes the issue by adding needed method of `Label(name string) string`.

This fix fixes 30279.

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>

Zebrilee/cli,docker/cli,riyazdf/cli,thaJeztah/cli,cpuguy83/cli,riyazdf/cli,redpanda/cli,dnephin/cli,ashfall/cli,docker/cli,dnephin/cli,thaJeztah/cli,e11137/cli,ashfall/cli,redpanda/cli,e11137/cli,allencloud/cli,gdevillele/cli,allencloud/cli,Zebrilee/cli,cpuguy83/cli