Skip creating new image if env is empty

[froody]

The docker driver always creates a new container + image in Setup, which for large images causes a long delay during Destroy . For my image with a simple test that checks for the exit code, skipping the new image creation when there is no env saves 3min of execution time:

Before: container-structure-test -v debug test -c -i 0.03s user 0.03s system 0% cpu 3:26.83 total
After: container-structure-test -v debug test -c -i 0.03s user 0.01s system 0% cpu 14.933 total