build/docker: use build context from pre-saved LLVM container to avoid rebuilding

Signed-off-by: deadprogram <ron@hybridgroup.com>

.github/workflows/docker.yml

@@ -53,6 +53,7 @@ jobs:
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          build-contexts: tinygo-llvm-build=docker-image://tinygo/llvm-15
           cache-from: type=gha
           cache-to: type=gha,mode=max
       - name: Trigger Drivers repo build on Github Actions