I docker rmi all the images such that they would repull:
[root@zuul-2 examples] (ibm-repo-1) > docker images list
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
[root@zuul-2 examples] (ibm-repo-1) > docker images
REPOSITORY                TAG                 IMAGE ID            CREATED             SIZE
examples_logs             latest              7ba511091c5f        About an hour ago   91.8 MB
examples_node             latest              d626e47ee140        About an hour ago   301 MB
zuul/nodepool-launcher    latest              47bf27b07cfa        20 hours ago        195 MB
zuul/zuul                 latest              d0d125ce297f        2 days ago          265 MB
zuul/zuul-scheduler       latest              d60a9ba4dee0        2 days ago          265 MB
zuul/zuul-executor        latest              e290ca67f6ef        2 days ago          312 MB
zuul/zuul-web             latest              e3f96550028f        2 days ago          265 MB
<none>                    <none>              28b992945c3d        11 days ago         91.8 MB
httpd                     alpine              45c9b7b78130        2 weeks ago         91.8 MB
zookeeper                 latest              f336949ce7a1        2 weeks ago         148 MB
mariadb                   latest              67238b4c1da0        3 weeks ago         365 MB
gerritcodereview/gerrit   latest              3440cfd08b41        4 weeks ago         956 MB
rastasheep/ubuntu-sshd    latest              49533628fb37        5 months ago        234 MB

Error traceback from the examples_executor_1:
xecutor_1      | 2018-11-09 21:59:03,027 WARNING zuul.Repo: Retry 2: Clone /var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config
executor_1      | 2018-11-09 21:59:03,225 ERROR zuul.ExecutorServer: Exception in update thread:
executor_1      | Traceback (most recent call last):
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/executor/server.py", line 2279, in _updateLoop
executor_1      |     self._innerUpdateLoop()
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/executor/server.py", line 2299, in _innerUpdateLoop
executor_1      |     task.branches = repo.getBranches()
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/merger/merger.py", line 298, in getBranches
executor_1      |     repo = self.createRepoObject()
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/merger/merger.py", line 235, in createRepoObject
executor_1      |     self._ensure_cloned()
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/merger/merger.py", line 163, in _ensure_cloned
executor_1      |     self._git_clone(clone_url)
executor_1      |   File "/usr/local/lib/python3.7/site-packages/zuul/merger/merger.py", line 195, in _git_clone
executor_1      |     kill_after_timeout=self.git_timeout)
executor_1      |   File "/usr/local/lib/python3.7/site-packages/git/cmd.py", line 548, in <lambda>
executor_1      |     return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
executor_1      |   File "/usr/local/lib/python3.7/site-packages/git/cmd.py", line 1014, in _call_process
executor_1      |     return self.execute(call, **exec_kwargs)
executor_1      |   File "/usr/local/lib/python3.7/site-packages/git/cmd.py", line 825, in execute
executor_1      |     raise GitCommandError(command, status, stderr_value, stdout_value)
executor_1      | git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
executor_1      |   cmdline: git clone ssh://git@github.ibm.com/wrigley/zuul-experiment-config.git /var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config
executor_1      |   stderr: 'Cloning into '/var/lib/zuul/executor-git/github.ibm.com/wrigley/zuul-experiment-config'...
executor_1      | Host key verification failed.
executor_1      | fatal: Could not read from remote repository.
executor_1      |
executor_1      | Please make sure you have the correct access rights
executor_1      | and the repository exists.'

==============================================
When I do the same using the key from the same system:
/usr/src # ssh-agent sh -c 'ssh-add /opt/zuul_keys/github.ibm.com/id_rsa; git clone ssh://git@github.ibm.com/wrigley/zuul-experiment-c
onfig.git'
Identity added: /opt/zuul_keys/github.ibm.com/id_rsa (/opt/zuul_keys/github.ibm.com/id_rsa)
Cloning into 'zuul-experiment-config'...
The authenticity of host 'github.ibm.com (169.60.70.162)' can't be established.
ECDSA key fingerprint is SHA256:ZkDIk3kbswojRjqiSBsXS6xOHF+y+9nzcbZypokADKA.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.ibm.com,169.60.70.162' (ECDSA) to the list of known hosts.
remote: Counting objects: 14, done.
remote: Total 14 (delta 0), reused 0 (delta 0), pack-reused 14
Receiving objects: 100% (14/14), done.
/usr/src # ls
zuul-experiment-config

=================================================
IMHO: I'm wondering if the call to the Repo class has its sshkey set?