Add Gitea Actions workflow to build/push the server image

Builds server/Dockerfile and pushes to this repo's Gitea Container
Registry (git.thumeit.com/tfaour/espresso-frame-server) on every push to
main that touches server/, tagged both latest and the commit SHA.

docker-compose.yml now sets both image: and build: -- deploy hosts can
docker compose pull to grab the CI-built image without needing this
repo's build context, while local dev can still docker compose build
against Dockerfile changes directly.

server/README.md

@@ -41,6 +41,23 @@ algorithm itself -- it just streams the response straight to the panel.
   color primaries for this panel) -- tune them once you can compare a
   rendered test image against the real panel.
 
+## Deploying a pre-built image
+
+Every push to `main` that touches `server/` triggers a Gitea Actions
+workflow (`.gitea/workflows/server-docker-build.yml`) that builds this
+image and pushes it to this repo's Gitea Container Registry at
+`git.thumeit.com/tfaour/espresso-frame-server`. `docker-compose.yml`
+already points at that image, so a deploy host doesn't need this repo's
+build context at all -- just the compose file:
+
+```
+docker compose pull
+docker compose up -d
+```
+
+`docker compose build` (or `up --build`) still works too, for local
+iteration against your own Dockerfile changes.
+
 ## Local development (without Docker)
 
 ```