github: workflow: build-container: don't hardcode repo

.github/workflows/build-container.yml

@@ -12,7 +12,7 @@ on:
 
 env:
   REGISTRY: ghcr.io
-  IMAGE_NAME: gluon-build
+  IMAGE_NAME: ${{ github.repository }}-build
 
 jobs:
   build-and-push-image:
@@ -36,7 +36,7 @@ jobs:
         id: meta
         uses: docker/metadata-action@e6428a5c4e294a61438ed7f43155db912025b6b3
         with:
-          images: ${{ env.REGISTRY }}/freifunk-gluon/${{ env.IMAGE_NAME }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
       - name: Build and push Docker image
         uses: docker/build-push-action@4a13e500e55cf31b7a5d59a38ab2040ab0f42f56
         with: