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 <yong.tang.github@outlook.com>
a2afbcbb57