Make it clear that a dry run is being executed in the log output

I got fooled by my own infra logic. After running a test build, I was expecting the images to be published to our dotnetdocker ACR under a "test" repository but never saw them show up. I was worried something was wrong with ACR. But it finally made sense when it was revealed that, because the "test" repo was being targeted, the infra automatically executes a dry run, preventing any images from actually getting published to that repo.

In order to help avoid false alarms like this, it would be beneficial to include clear messages that a dry run is happening. Probably every time an operation is being done that has side effects, it should include "DRY RUN" in the header output. Something like this:

```
COPYING IMAGES (===> DRY RUN <===)
--------------
Importing 'test/dotnet/core-nightly/runtime-deps:2.1-stretch-slim' from 'build-staging/688415/dotnet/core-nightly/runtime-deps:2.1-stretch-slim'
Importing 'test/dotnet/core-nightly/runtime-deps:2.1.19-stretch-slim' from 'build-staging/688415/dotnet/core-nightly/runtime-deps:2.1.19-stretch-slim'
Importing 'test/dotnet/core-nightly/runtime-deps:2.1-alpine3.11' from 'build-staging/688415/dotnet/core-nightly/runtime-deps:2.1-alpine3.11'

```