`SOURCE_DATE_EPOCH` is not respected

[Pesa]

Description

metadata-action does not seem to honor the SOURCE_DATE_EPOCH env variable when generating labels and annotations such as org.opencontainers.image.created. I know these can be overridden by the calling workflow, but it would be nice if the action supported SOURCE_DATE_EPOCH by default, for a better out-of-the-box experience.

I'm not sure if SOURCE_DATE_EPOCH should also affect tags that use the {{date}} expression. Personally I don't think it should. Maybe a new expression {{source_date_epoch}} can be added that takes the timestamp from the env variable and formats it as specified, instead of using the current time, but this can be considered separately later.

[crazy-max]

Sounds good thanks