Jenkinsfile: set repo and branch for DCO check as well

Commit 7019b60 added these
env-vars to other stages, but forgot to update the DCO stage,
which also does a diff to validate commits that are in a PR.

Also adding openssh-client, for situations where the upstream
needs to be accessed through an ssh connection.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 7c5fd83)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Jenkinsfile

@@ -47,7 +47,9 @@ pipeline {
                 sh '''
                 docker run --rm \
                   -v "$WORKSPACE:/workspace" \
-                  alpine sh -c 'apk add --no-cache -q git bash && cd /workspace && hack/validate/dco'
+                  -e VALIDATE_REPO=${GIT_URL} \
+                  -e VALIDATE_BRANCH=${CHANGE_TARGET} \
+                  alpine sh -c 'apk add --no-cache -q bash git openssh-client && cd /workspace && hack/validate/dco'
                 '''
             }
         }