Add DCO check via PullApprove

Closes #516
PullApprove has support to check for DCO
Also, take the time to improve the approve_regex

Signed-off-by: Chris Aniszczyk <[email protected]>

Author: caniszczyk

.pullapprove.yml

@@ -3,6 +3,8 @@ approve_regex: ^LGTM
 reject_regex: ^Rejected
 reset_on_push: true
 author_approval: ignored
+signed_off_by:
+  required: true
 reviewers:
   teams:
   - runtime-spec-maintainers