Migration of the container images to the Github Container Registry

[fmartingr]

Thanks to @n8225 's help we have automated container images hosted in github! Thanks a lot!

From now on, the official and supported docker images are found here.

The latest stable Shiori image will be tagged as latest, and the last unstable version (the latest commit on the master branch) will be tagged dev.

If you have any questions regarding this please use the Q&A forum.