fix gitea missing expressions

Signed-off-by: kaedwen <kaedwen@heinrich.blue>

.gitea/workflows/docker.yaml

@@ -14,7 +14,6 @@ on:
       - master
 
 env:
-  REGISTRY: gitea.heinrich.blue
   IMAGE_NAME: ${{ gitea.repository }}
 
 jobs:
@@ -22,9 +21,19 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: read
-      packages: write
+      packages: write # not supported yet in gitea ...
 
     steps:
+      - name: Set registry from instance URL
+        run: echo "REGISTRY=${SERVER_URL#*://}" >> "$GITHUB_ENV"
+        env:
+          SERVER_URL: ${{ gitea.server_url }}
+
+      - name: Set lowercase image name
+        run: echo "IMAGE_NAME=${REPO,,}" >> "$GITHUB_ENV"
+        env:
+          REPO: ${{ gitea.repository }}
+
       - name: Checkout code
         uses: actions/checkout@v4