Add ms-azuretools.vscode-containers

[Alanaktion]

• I have read the note above about PRs contributing or fixing extensions
• I have tried reaching out to the extension maintainers about publishing this extension to Open VSX (if not, please create an issue in the extension's repo using this template).
  • I do not expect that Microsoft would do this themselves for this extension, and have not created an issue there for that reason.
• This extension has an OSI-approved OSS license (we don't accept proprietary extensions in this repository)

Description

This Container Tools Extension is replacing Microsoft's Docker extension, see microsoft/vscode-docker#4559

#861 also exists to add this extension, but is missing the build step.

[filiptronicek]

@Alanaktion thanks for you contribution. Everything looks good, so let's ship it.