Simple summary logging of images pulled during execution

[rnorth]

Another small mitigation for #3099: so that users have better visibility over number of images pulled during builds.

Produces an output like:

13:17:07.689 INFO  org.testcontainers.utility.ImagePullCountLogger - Testcontainers pulled the following images during execution:
    postgis/postgis:12-3.0
    postgres:10-alpine
    postgres:9.6.12
    postgres:9.6.8

Conversely if no images have been pulled, Testcontainers did not need to pull any images during execution will be logged (for avoidance of doubt).

For this change:

• Added a singleton to keep track of images that have been pulled
• Wired in the LoggedPullImageResultCallback mechanism to record completed pulls

I'm using String image names for this, but will raise a separate PR to refactor to use DockerImageName (that's a bit of a sprawling change).

[rnorth]

Using a JVM shutdown hook. This is not the most iron-clad way, but as this logging is informational and not required I believe it's sufficient.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you believe this is a mistake, please reply to this comment to keep it open. If there isn't one already, a PR to fix or at least reproduce the problem in a test case will always help us get back on track to tackle this.

[rnorth]

I think for the sake of tidiness I should close this now.