IT-3421: Add vulnerability scanning to container build

[brucehoff]

• Builds container and caches it as a '.tar' file
• Passes the '.tar' file to Trivy for vulnerabilty checking
• Vul'n findings appear under the GitHub "Security" tab
• Only pushes container image to GHCR if vul'n scan passes
• Pushes to multiple tags, including Major.minor.patch and Major.minor, allowing the update of Major.minor for security fixes
• Provides the ability to suppress "false positive" findings
• Adds a daily scan, to identify new vul'ns in existing image
• Documents how vul'n scanning and versioning work in README
• Updates the base image from RStudio 4.3 to 4.4 (to address identified vulnerabilities)

Note: We increase the 80 character limit to 200 in .yaml files because in trivy.yml we need to list files to skip, some of which have path lengths > 80 characters, and breaking them over multiple lines is unwieldy.

[github-advanced-security]

This pull request sets up GitHub code scanning for this repository. Once the scans have completed and the checks have passed, the analysis results for this pull request branch will appear on this overview. Once you merge this pull request, the 'Security' tab will show more code scanning analysis results (for example, for the default branch). Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results. For more information about GitHub code scanning, check out the documentation.