ADD learned to cache it's output layer

[mzihlmann]

Description

So far ADD instructions were not cacheable, only COPY instructions. This is not really a blocker as most operations can and should be done with COPY instead, but nevertheless it's confusing.

With this change we implement caching analogous to COPY instructions. No special precautions for tar unpacking etc has to be taken as they only affect the initial build, not the rebuild from cache.

Submitter Checklist

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Examples of user facing changes:
- ADD learned to cache it's output layer