Add CI job that verifies sidecar images digests in devfile registry

[mkuznyetsov]

Is your task related to a problem? Please describe.

Occasionally, images that we list for sidecars may have outdated digests. If that happens, we would like to find out about it as early as possible (before the scheduled weekly releases), and decide whether we should update the images

Describe the solution you'd like

A github actions workflow that runs daily to check if all images have correct digest, if not - fetch a correct one (with skopeo or curl) and submit a PR to devfile-registry to update it

Describe alternatives you've considered

Additional context

[nickboldt]

Specific reason for this is the quarkus sidecar image, which keeps updating its floating tags and removing older images' digests.

[nickboldt]

WIP in eclipse-che/che-devfile-registry#380 awaits revision and merge.

[nickboldt]

Job has failed in https://github.com/eclipse-che/che-devfile-registry/runs/2471789961?check_suite_focus=true (12 hrs ago)

 ./arbitrary-users-patch/check_sidecar_images_digests.sh
  shell: /usr/bin/bash -e {0}
/home/runner/work/_temp/b0039ca4-c0f6-46d5-80be-30920f8bb6fd.sh: line 4: ./arbitrary-users-patch/check_sidecar_images_digests.sh: No such file or directory
Error: Process completed with exit code 127.

[nickboldt]

WIP in https://github.com/eclipse-che/che-devfile-registry/pull/383/files

[nickboldt]

Done and green in https://github.com/eclipse-che/che-devfile-registry/actions/workflows/sidecar-check.yml