Upgrade release Dockerfile from Ubuntu 16.04 to 20.04 by bergmeister · Pull Request #1631 · PowerShell/PSScriptAnalyzer

PR Summary

This is to prepare for Ubuntu 16.04 reaching EOL at the end of April 2021. My understanding is that this file is used in the release process but I cannot see a reference to it, therefore please adapt to the path if needed, I improved the path to not be specific to the ubuntu version any more.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.