How do I connect a Spaces account to my Plex server. For spaces, there is a URL: spacename.region.digitaloceanspaces.com
I guess I need to mount it somehow to my Plex server? My Plex is running in a Docker container on a Ubuntu 16.04 server.
You could mount your Space as a directory using s3fs, see https://mlohr.com/mount-digitalocean-spaces-linux/. However, for this specific use case it's recommended to use Block Storage instead.
Out of curiosity, why would Block Storage be recommended for this use-case? I'm new to this and was actually thinking of going this exact route. However, for the capacity I need block-storage would be rather costly.
With programs like Plex that expect a filesystem and don't natively support object storage, you will get much better performance with block storage as it's basically a virtual disk attached to your Droplet. Object storage isn't really meant to be used as a traditional filesystem. Mounting an object storage bucket as a directory is usually slow overall (regardless of the service providing it) compared to a regular filesystem, so there's a possibility that it wouldn't be suitable for streaming videos.
Another very important thing to consider is bandwidth cost. If you're streaming videos directly out of a Space without any kind of caching you will quickly use up a huge amount of bandwidth.
You can definitely try it out, see how it goes, and evaluate!