[new feature] Auto-deploy preview server with a workflow_dispatch action

[bmispelon]

Now that #2153 is merged we're well on our way to have a preview server where we can test features in close-to-prod conditions.

The current solution is not ideal however, because a lot of it is still quite manual. Things should ideally be more automated.

One idea raised in the comments of the "preview branch" PR was to switch to a workflow_dispatch github action and combine that with some form of automatic deployment. I think that's a good idea.

The way I understand there are two parts to this feature:

• Replacing the current preview branch tagging github action (see Changed GH action to publish images from the preview branch too #2153) with a manually triggered workflow_dispatch. This could be done by any contributor
• Setting up an automatic deployment on the preview server. This will need to be done by someone from the ops team (likely me), but the technical approach could be discussed in this ticket