docker: Keep LF on bash configs for Windows builds

Fixes an issue when building on Windows where the bash configs
(`docker/etc/skel`) were checked out with `\r\n` file endings, then
included with `COPY` during docker build. Causing an error when running
the container:

```bash
bash: $'\r': command not found
bash: /home/pi/.bashrc: line 6: syntax error near unexpected token
`$'in\r''
'ash: /home/pi/.bashrc: line 6: `case $- in
```

Everything in the docker/rpi folder should be treated as Linux files.

docker/rpi/.gitattributes

@@ -0,0 +1,2 @@
+# All files in this folder are taken from a Raspberry Pi so they should always use Linux file endings
+* text eol=lf