Upgrade gitea to 1.13.6

This picks up a variety of bug fixes and ensures we're keeping up.
The diff of the template files we modify between gitea v1.13.1, v1.13.4,
and v1.13.6 is empty. The diff between our modifications at v1.13.4
looks about how I would expect (implying that v1.13.6 is also fine).
Reviews should double check though.

We also add in setup for the system-config repo on the test gitea as
this will give us something to look at for verification purposes.

Change-Id: Idb3568a9d287a2d46d568ab7d8d3a7108739d23e

docker/gitea/Dockerfile

@@ -26,7 +26,7 @@ FROM docker.io/library/golang:1.15-buster AS build-env
 
 LABEL maintainer="infra-root@openstack.org"
 
-ARG GITEA_VERSION="v1.13.1"
+ARG GITEA_VERSION="v1.13.6"
 ENV TAGS "bindata $TAGS"
 
 #Build deps

playbooks/test-gitea.yaml

@@ -16,3 +16,13 @@
       fail:
         msg: Zuul description was not updated
       when: '"Zuuls new description" not in zuul_content.content'
+    - name: Push system-config into our test gitea for testing
+      shell: |
+        cd /home/zuul/src/opendev.org/opendev/system-config
+        git remote add test-gitea https://gerrit:{{ gitea_gerrit_password }}@localhost:3000/opendev/system-config
+        GIT_SSL_NO_VERIFY=1 git push -f test-gitea master
+    # This is conveniently left here so that it can be uncommented in order to
+    # autohold the system-config-run-gitea job in zuul.
+    #- name: Force a failure for human intervention
+    #  fail:
+    #    msg: Failing so that we can check on things with a hold