Update dockerfile path for functional tests

Author: shaun-nx

.github/workflows/functional.yml

@@ -107,7 +107,7 @@ jobs:
       - name: Build NGINX Docker Image
         uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6.18.0
         with:
-          file: build/Dockerfile${{ inputs.image == 'nginx' && '.nginx' || '' }}${{ inputs.image == 'plus' && '.nginxplus' || ''}}
+          file: build${{ inputs.build-os != '' && format('/{0}', inputs.build-os) || '' }}/Dockerfile${{ inputs.image == 'nginx' && '.nginx' || '' }}${{ inputs.image == 'plus' && '.nginxplus' || '' }}
           tags: ${{ steps.nginx-meta.outputs.tags }}
           context: "."
           load: true