Make local build script spin up a temporary registry

.github/workflows/build.yml

@@ -121,6 +121,7 @@ jobs:
         run: |
           IMG="ghcr.io/${GITHUB_REPOSITORY,,}/${{ matrix.target }}-${{ matrix.variant }}"
           echo "::set-output name=name::${IMG/ /-}"
+          echo "::set-output name=gh_repo::${GITHUB_REPOSITORY,,}"
       - name: Stop Commands
         run: T="$(echo -n ${{ github.token }} | sha256sum | head -c 64)" && echo -e "::add-mask::${T}\n::stop-commands::${T}"
       - name: Build target base image
@@ -132,6 +133,8 @@ jobs:
           tags: ${{ steps.imagename.outputs.name }}:latest
           cache-to: type=registry,mode=max,ref=${{ steps.imagename.outputs.name }}:cache
           cache-from: type=registry,ref=${{ steps.imagename.outputs.name }}:cache
+          build-args: |
+            GH_REPO=ghcr.io/${{ steps.imagename.outputs.gh_repo }}
   build_ffmpeg:
     name: Build ffmpeg
     if: ${{ ( github.event.inputs.buildOnly == 'true' && !cancelled() ) || success() }}