Bump actions/cache from 2 to 2.1.5 (#230)

Bumps [actions/cache](https://github.com/actions/cache) from 2 to 2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v2...v2.1.5)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/push.yml

@@ -18,7 +18,7 @@ jobs:
         with:
           install: true
       - name: Cache Docker layers
-        uses: actions/cache@v2
+        uses: actions/cache@v2.1.5
         with:
           path: /tmp/.buildx-cache
           key: ${{runner.os}}-buildx-${{github.sha}}