The DCO job runs inside an Alpine container and fetches the base
branch from VALIDATE_REPO unless VALIDATE_ORIGIN_BRANCH is set.
In forked runs that repository can point at a private upstream URL, so
the unauthenticated fetch fails with:
```
fatal: could not read Username for 'https://github.com': No such
device or address
```
Pass the pull request base SHA so the validator can resolve the
comparison point locally.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
Paweł Gronowski
78c2aa4fcc