Issues with viewport using backstopJS docker image

Hey,

I came across some issues during implementation of visual regression tests.

* The given height gets ignored on some pages, so the full height of the page gets tested.
* When emulating the iphone6 sometimes randomly an additional margin is added to the bottom of the page so the test will be marked as failure when same dimensions are required.

Any ideas how to solve these problems?

I am using the backstopJS/backstopJS docker image v3.0.37