1.7.1 - Hotfix for saving thumbnail

Fixes

• Correctly save thumbnail to disk, filepath is /recordings/thumbnails//latest_thumbnail.jpg, fixes #91

Docker images are available on Docker Hub
roflcoopter/viseron:1.7.1
roflcoopter/viseron-cuda:1.7.1
roflcoopter/viseron-vaapi:1.7.1
roflcoopter/viseron-rpi:1.7.1