Update gradle.yml

Author: daggerok

.github/workflows/gradle.yml

@@ -26,7 +26,7 @@ jobs:
         uses: actions/cache@v1
         with:
           path: ~/.docker
-          key: ${{ runner.os }}-${{ matrix.os }}-docker-
+          key: ${{ runner.os }}-${{ matrix.os }}-docker-${{ hashFiles('**/Dockerfile*') }}
           restore-keys: |
             ${{ runner.os }}-${{ matrix.os }}-docker-