Suppress `DOCKER_METADATA_OUTPUT` environment variables output option

[kriswuollett]

Description

An option to to suppress DOCKER_METADATA_OUTPUT environment variables output from the action would be great if they are not needed downstream.

Besides, actions/runner#1126, when unneeded they add many unnecessary lines of output to the actions logs for each step.

[crazy-max]

I think that's a thing that needs to be addressed on runner side imo.

Edit: Ah that's about the env vars not the action's outputs. Then yeah I think we could do something about it.