Force cache upload if it changed

We build our container on php:7.2 base image. That means that every time a patch version is released, the base image is changed. That happens quote often. We're using docker cache for our build.

That works great. First build takes a longer time as it compile PHP extensions, etc. Subsequent builds are using the cached layers to speed up the process.

But when a new patch version of the base image is released, the base image changes. Therefore build pulls whole new image and all the previously cached layers are invalid. It builds the image from scratch and then when build succeeds it does not proceed to upload the cache with following message.

**Cache "docker": Skipping upload for existing cache**
Searching for test report files in directories named [test-results, failsafe-reports, test-reports, surefire-reports] down to a depth of 4
Finished scanning for test reports. Found 0 test report files.
Merged test suites, total number tests is 0, with 0 failures and 0 errors.

Currently the only workaround I know is to delete the cache every time new patch version of the base image is released.