Improve default experience with Devcontainers

Using [Devcontainers](https://code.visualstudio.com/docs/remote/containers) is a nice way to develop when there are issues with VSCode being sandboxed. Getting Devcontainers to work from the VSCode Flatpak is easy. Just create a simple wrapper script

```sh
#!/bin/sh
exec flatpak-spawn --host docker "$@"
```

and point the Devcontainer extension to it.

I suggest to ship this wrapper with the VSCode Flatpak, so that the Devcontainer extension works out of the box. Having the same wrapper for Podman might be a good idea too. Another option would be to describe this workaround in `flatpak-warning.txt`.