change base image to our own image
change base image to our own image

we were using an unsupported image previously
we want to make sure that the base image we use for testing is the same for building

[#162284557]
[#163591178]

Signed-off-by: Andreas Voellmer <1dc0180bbd6432aaf67635695469844eef57b15e@pivotal.io>

pivotal-cf-experimental/pivnet-resource,pivotal-cf-experimental/pivnet-resource