ci: push to GHCR from the Gitea workflow instead of a separate one

The Gitea instance's act_runner is the CI that actually executes here;
GitHub Actions on the mirror isn't confirmed to run at all (and a
push-mirror lacking the "workflow" PAT scope can silently drop
.github/workflows changes anyway). Add a GHCR login/push to the
existing .gitea/workflows/docker-publish.yml job instead, so one
build produces tags on both the Gitea registry and ghcr.io. Drop the
now-redundant .github/workflows/docker-publish.yml.

Requires a new repo secret GHCR_TOKEN: a GitHub PAT with write:packages
scope (separate from REGISTRY_TOKEN, which is scoped to the Gitea
registry).

.gitea/workflows/docker-publish.yml

@@ -27,6 +27,13 @@ jobs:
           username: ${{ github.actor }}
           password: ${{ secrets.REGISTRY_TOKEN }}
 
+      - name: Log in to GHCR
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GHCR_TOKEN }}
+
       - name: Build and push
         uses: docker/build-push-action@v6
         with:
@@ -35,3 +42,5 @@ jobs:
           tags: |
             ${{ vars.REGISTRY_HOST }}/${{ env.IMAGE_NAME }}:latest
             ${{ vars.REGISTRY_HOST }}/${{ env.IMAGE_NAME }}:${{ github.sha }}
+            ghcr.io/${{ env.IMAGE_NAME }}:latest
+            ghcr.io/${{ env.IMAGE_NAME }}:${{ github.sha }}