Upgrade release Dockerfile from Ubuntu 16.04 to 20.04

[bergmeister]

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.
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