Add CI workflow for building and releasing server docker images

[Robert-Steiner]

Issue

Add CI workflow for building and releasing server docker images

Description

The PR adds a reusable workflow to build server docker images for various platforms. The images are pushed to dockerhub. Currently the workflow can only be triggered via a workflow_dispatch event. There are two ways to create such an event. You can use UI to create one or you can use the gh cli tool. Later the workflow will also run as part of the release workflow.

The workflow builds and releases images for arm64 and amd64. We are currently not releasing arm/v7 due to difficulties compiling the cryptography package (see pyca/cryptography#7891). The default python version for the server image is 3.11.

Any other comments?

• Dockerhub does not support immutable tags. Therefore it is possible to override an existing tag. Should we add a check to prevent that?
• Later we can also add the possibility to publish images via a custom tag. Might be useful if we want to test a certain branch, feature, etc.
• I was not able to test the workflow_dispatch event because we can only trigger it once it is merged.
• workflow run: https://github.com/adap/flower/actions/runs/7102820035
• https://hub.docker.com/r/flwr/server/tags

[tanertopal]

lgtm!