skaffold build fails in a non-git repo

[dgageot]

When getting started with Skaffold, we might run skaffold in a folder that doesn't have git information yet. In that case, the build step will fail with this error:

FATA[0017] build step: generating tag: opening git repo: repository does not exist

We might want to do something anyway. Either fallback to another tagger or just create a digest out of files in the folder

[dgageot]

cc @guillaumerose