Date label should be applied last

[bjj]

I'm using the docker plugin with cache_from to make a build environment. It never changes, so literally half of my build time for that step is applying labels and then pushing those layers back to the registry. It would be nice if the only variable layer were applied last so all of the rest could be cached.

(I'm not sure why the labeling is so slow in general)

[gtg3vv]

I'm also using cache_from for a rarely changed image, except that I would prefer to be able to turn off this label entirely to avoid unnecessary cache busting.

[bradrydzewski]

sounds like a good request. If someone can send a PR, we can take a look and merge quickly.

[gtg3vv]

Done: #297