* dockerfile: use multistage build, add dockerfile to ignore

* gdrive: supports only oauth 2.0 client id json file for now

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
```

* implemented docker
* added previews (markdown, html, text, video, audio)
* added tor support
* several bugfixes