dockerfile: promote experimental linter rule

[jsternberg]

This promotes the CopyIgnoredFile linter rule out of experimental.

[jsternberg]

Modified this PR to only promote copy ignored file. We had some off github conversations about this and decided InvalidDefinitionDescription was going to be too noisy to remove from experimental. We still think there's value to the check, but we think it'll be too noisy to ever promote out of experimental and we also don't want to keep it in experimental forever. We might create a new category of strict or pedantic checks to allow them to optionally be turned on for specific dockerfiles.