Switch to alpine base image
Switch to alpine base image

This drastically decreases the overall image size. Quick comparison:

```
# Image fetched from Docker Hub
dutchcoders/transfer.sh   latest              a875ed5dfa56        5 months ago        704MB
# Image just built
dutchcoders/transfer.sh   latest              a220c49297eb        4 minutes ago       269MB
```
dutchcoders/transfer.sh,dutchcoders/transfer.sh