Include docker image pull command to prevent https://github.com/schlagmichdoch/PairDrop/issues/33

docs/host-your-own.md

@@ -99,6 +99,10 @@ The easiest way to get PairDrop up and running is by using Docker.
 
 ### Build the image
 ```bash
+docker image pull node:lts-alpine
+```
+> Prevents Docker from using an old `node:lts-alpine` image by pulling the latest
+```bash
 docker build . -f Dockerfile -t pairdrop
 ```
 > A GitHub action is set up to do this step automatically