chore: Ignore `.devcontainer` folder

[VSCode dev containers](https://code.visualstudio.com/docs/remote/containers)
allows to develop in a container and is configured through files
in `.devcontainer`. However, that directory falls in the same
category as `.vscode` in that it might be different on each
developer's machine, depending on preference and platform he's
working on. Therefore we exclude this directory similar to
`.vscode`.