AzureML deployment issue - OpenCV dependencies

I am recieving the following error when trying to deploy a model with my entry script that uses opencv.

`ImportError: libGL.so.1: cannot open shared object file: No such file or directory`

I understand this is because the docker image behind the scenes doesn't contain the following lines:
`RUN apt-get update `
`RUN apt-get install ffmpeg libsm6 libxext6 -y`

How can add this to the docker image automatically created during deployment?

Thanks,