Tasks: Support for layer caches

**Is this a BUG REPORT or FEATURE REQUEST?**:

Feature request

**What happened?**:

I’m building an image via quick task. It appears that steps in my Dockerfile that create intermediate layers are not re-used on subsequent builds of the same Dockerfile.

**What did you expect to happen?**:

I would like to have ACR Tasks use layer caches from previous build to skip build time of those layers.

**How do you reproduce it (as minimally and precisely as possible)?**:

Build an image with a Dockerfile.

Build the same Dockerfile again: steps that should be cached are run again unnecessary.