Update workflow.yml to include Postgres 17

[Kuchenmampfer]

I am just guessing that this change will make the ci produce an image for Postgres 17 as well. Feel free to correct me if other things need to be done for that as well.

That support is needed, because the MASH-Playbook already deploys Postgres 17 which currently can not be backed up (via the borg(matic) role). I already made an issue there. Today, I manually set my Postgres version to 17 and noticed that there is no image available here.

[aine-etke]

Unfortunately, that's not how it works.
Supported postgres versions depend on the actual package's availability in alpine linux, so the proposed PR won't work until the v17 package is available.