Add gnupg2 to the SageMaker Distribution

For organizations that [sign git commits with GPG](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification), it would be helpful if GPG was installed in the SageMaker distribution by default?

For now, I've got it working by adding an entry to the SMStudio lifecycle configuration script like below, to install at run time:

```sh
if ! command -v gpg &> /dev/null
then
echo "gpg not found: Trying to sudo apt-get install gnupg2"
sudo apt-get install gnupg2
fi
```

The issue names the SMStudio lifecycle configuration script and shows the runtime gpg installation command, but no repository file or test. Start by locating the SageMaker distribution's Docker image build and package-installation entry points. Done means the distribution provides gpg by default rather than requiring the reported runtime workaround.

Written by the indexing model from the issue text.