fix: Normalise `*.sh` File Endings (#16608)

Windows users encounter problems running the `docker-compose` commands
 to run an instance of Superset.

The reason for this is line-endings for the `*.sh` files that are
 mounted and used in the `command` sections for the superset images.

Many Windows users will check the repository out with line endings set
 to `CRLF`. When mounted in to the *nix-based containers, these line
 endings are not understood, leading to errors such as:

```text
/usr/bin/env: bash\r: No such file or directory
```

The addition of a `.gitattributes` file resolves this issue by telling
 git to always resolve these files with `LF` line endings.

+semver: fix
+ref: #10132