What are the differences between this extension and the remote-containers extension?

[abceleung]

Interestingly, both are created by Microsoft.

Are they supposed to be used together?

[bwateratmsft]

If you're referring to the Docker extension, then this and that are one and the same, because this is the repo for that extension.

[abceleung]

Oops, I meant the Remote - Containers extension.
https://github.com/Microsoft/vscode-remote-release

[bwateratmsft]

Gotcha. Remote - Containers is used for attaching VSCode to containers via VSCode remoting. You can read more about it here: https://code.visualstudio.com/docs/remote/containers