Set image digest in output

I need to have pushed image digest so it can be signed with [cosign](https://github.com/sigstore/cosign).
Example action step:
```
- name: Sign the published Docker image
if: ${{ github.event_name != 'pull_request' }}
env:
TAGS: ${{ steps.meta.outputs.tags }}
DIGEST: ${{ steps.build-and-push.outputs.digest }}
# This step uses the identity token to provision an ephemeral certificate
# against the sigstore community Fulcio instance.
run: echo "${TAGS}" | xargs -I {} cosign sign --yes {}@${DIGEST}
```