Deploy webui through OCI image

[Jip-Hop]

When deploying the stalwart OCI image from Docker Hub, the webui needs to be downloaded separately. It would be great if deploying the webui could be done by means of an OCI image, to keep the workflow consistent with deploying the stalwart image.

Options:

• Include the webui inside the stalwart image, no separate download
• Make a dedicated webui image, which could be mounted into the stalwart container using volume.type=image
• Running webui as a dedicated container, which communicates with the stalwart container (e.g. via network or shared mount)

This would potentially fix #1567 and #533 and #3020.