Fix variable reference for username in Docker login and metadata extraction steps

Signed-off-by: Stephen Simpson <ssimpson89@users.noreply.github.com>

.github/workflows/docker-image-linux.yml

@@ -26,14 +26,14 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: ${{ env.REGISTRY }}
-          username: ${{ env.USERNAME }}
+          username: ${{ vars.USERNAME }}
           password: ${{ secrets.PASS }}
 
       - name: Extract metadata for Docker
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/${{ env.USERNAME }}/${{ env.IMAGE_NAME }}
+          images: ${{ env.REGISTRY }}/${{ vars.USERNAME }}/${{ env.IMAGE_NAME }}
           tags: |
             type=raw,value=latest
             type=sha,format=short