Add gcc package requirment

[timorkal]

When trying to just use the syntax in the how to install "pip install sagemaker-training" I quickly got an error that gcc is missing in my base image. In such case I would recommend adding a requirements phase that are needed to be installed in the base image before running "pip install sagemaker-training". Also if there are other deps that are missing that I wasn't aware of (like the gcc) please add them also.