Fix container publish workflow to target the right registry per runner

The compose files were switched to pull from the Gitea registry
(git.us2plus2.com/marvin/vboxstock), but publish-container.yml was left
hardcoded to ghcr.io. Gitea Actions also executes .github/workflows, so
every push ran this job twice: it succeeded on GitHub (real
GITHUB_TOKEN, publishing to ghcr.io, which nothing consumes) and failed
on Gitea every time (Gitea's own GITHUB_TOKEN isn't valid for
github.com's registry). Now the registry/image target is derived from
github.server_url, so each runner logs into and publishes to its own
registry with its own automatic token.

.github/workflows/publish-container.yml

@@ -6,10 +6,6 @@ on:
     tags: ["v*"]
   workflow_dispatch:
 
-env:
-  REGISTRY: ghcr.io
-  IMAGE_NAME: mfwadejr/vboxstock
-
 jobs:
   publish:
     runs-on: ubuntu-latest
@@ -21,10 +17,22 @@ jobs:
       - name: Check out source
         uses: actions/checkout@v6
 
-      - name: Sign in to GitHub Container Registry
+      - name: Determine target registry
+        id: registry
+        run: |
+          if [ "${{ github.server_url }}" = "https://github.com" ]; then
+            echo "registry=ghcr.io" >> "$GITHUB_OUTPUT"
+            echo "image=ghcr.io/mfwadejr/vboxstock" >> "$GITHUB_OUTPUT"
+          else
+            host="$(printf '%s' '${{ github.server_url }}' | sed -E 's#^https?://##')"
+            echo "registry=$host" >> "$GITHUB_OUTPUT"
+            echo "image=$host/marvin/vboxstock" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Sign in to container registry
         uses: docker/login-action@v3
         with:
-          registry: ${{ env.REGISTRY }}
+          registry: ${{ steps.registry.outputs.registry }}
           username: ${{ github.actor }}
           password: ${{ secrets.GITHUB_TOKEN }}
 
@@ -32,7 +40,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          images: ${{ steps.registry.outputs.image }}
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
             type=ref,event=tag