more metadata tests and filesystem tests for docker tests

[orenmazor]

Hi all,

I'm migrating a test suite from googles container structure tests and I noticed terratest is missing two categories of tests:

1. I'd like to be able to do some metadata tests, such as confirming the user of an image. it looks like this change would just be extending the existing container inspect functionality: https://github.com/gruntwork-io/terratest/blob/master/modules/docker/inspect.go#L18

2. I'd like to be able to inspect files in my image. this can be sort of worked around by running your image with a cat /foo command, but my images have no shell (distroless bases), so this can't work for me. container structure tests uses the image -> tar functionality, so we could probably do the same thing: https://github.com/GoogleContainerTools/container-structure-test/blob/master/pkg/drivers/docker_driver.go#L186

thoughts?