Simplify devcontainer setup and resolve git line ending conflicts

[bergmeister]

PR Summary

• The .gitattributes change is so that VS-Code does not see red-herring diffs due to different line endings when using devcontainer (Remote Contaienr or GitHub CodeSpaces). This is taken from the docs here: https://code.visualstudio.com/docs/remote/troubleshooting#_resolving-git-line-ending-issues-in-containers-resulting-in-many-modified-files
• Remove some setup in the Dockerfile that is not necessary for PSSA development. The Dockerfile was originally taken from a generic .NET template for the new devcontainer feature, therefore I can now remove the old licence text. I believe that setup is more geared towards scenarios happen in ASP.NET development but not for the development of this module.

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.