Refactor docker build using build cache and multiline script

[glensc]

Replaces https://github.com/Stremio/server-docker/pull/40 (Fixes https://github.com/Stremio/server-docker/issues/41):

• Use JELLYFIN_VERSION argument instead of using hard coded value
• Detect OS version instead of hard coded value

Reduces size of image by omitting apt caches via cache build mounts. This also simplifies whole scripting by using multiline script.

Tested build with:

docker build . --progress=plain --build-arg VERSION=v4.20.1