Add nightly-build pipeline

Change-Id: I85ac89583726810fda18ec85e0b10437eff94c22
diff --git a/zuul.d/pipelines.yaml b/zuul.d/pipelines.yaml
index c2fcb76..84a32af 100644
--- a/zuul.d/pipelines.yaml
+++ b/zuul.d/pipelines.yaml
@@ -54,3 +54,13 @@
           scheme: 'zuul-check'
           state: FAILED
           message: 'Change failed'
+
+- pipeline:
+    name: nightly-build
+    description: |
+      Pipeline for nightly builds, executed everyday.
+    manager: serial
+    precedence: low
+    trigger:
+      timer:
+        time: 2 0 * * *