Improve documentation for using the Containerfile on the install page

[alilleybrinker]

We should improve this section: https://hipcheck.mitre.org/docs/getting-started/install/#using-the-containerfile

Specifically to reflect the need to pass through the HC_GITHUB_TOKEN environment variable and to specify a policy file.

An invocation with podman would look like:

$ podman run -e HC_GITHUB_TOKEN=$HC_GITHUB_TOKEN localhost/hc-image:latest --policy /home/hc_user/.config/hipcheck/Hipcheck.kdl check https://github.com/mitre/hipcheck

Assuming you've built the Containerfile and have it in your local image store.