use dep as denpendency managment tool

vendor/github.com/opencontainers/go-digest/.pullapprove.yml

@@ -0,0 +1,12 @@
+approve_by_comment: true
+approve_regex: '^(Approved|lgtm|LGTM|:shipit:|:star:|:\+1:|:ship:)'
+reject_regex: ^Rejected
+reset_on_push: true
+author_approval: ignored
+signed_off_by:
+  required: true
+reviewers:
+  teams:
+  - go-digest-maintainers
+  name: default
+  required: 2