Make .Net Core version in Docker image of .devcontainer match the one in global.json

[bergmeister]

PR Summary

.Net Core allows newer patched versions but not new minor version and since the 3.1 SDK is now on 3.1.2, we need to specify the exact version in order to be able to build in a container/codespace.

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.