feat: Add container to propagate host network MTU (#1201)

* feat: Add container to propagate host network MTU

Some network environments use non-standard MTU values. In these
situations, the `DockerMTU` setting might be used to specify the MTU
setting for the `bridge` network created by Docker. However, when the
Github Actions workflow creates networks, it doesn't propagate the
`bridge` network MTU which can lead to `connection reset by peer`
messages.

To overcome this, I've created a new docker image called
`summerwind/actions-runner-mtu` that shims the docker binary in order to
propagate the MTU setting to networks created by Github workflows.

This is a follow-up on the discussion in
(#1046)[https://github.com/actions-runner-controller/actions-runner-controller/issues/1046]
and uses a separate image since there might be some unintended
side-effects with this approach.

* fixup! feat: Add container to propagate host network MTU

Co-authored-by: Yusuke Kuoka <ykuoka@gmail.com>

TROUBLESHOOTING.md

@@ -256,8 +256,28 @@ spec:
       env: []
 ```
 
-There may be more places you need to tweak for MTU.
-Please consult issues like #651 for more information.
+If the issue still persists, you can set the `ARC_DOCKER_MTU_PROPAGATION` to propagate the host MTU to networks created
+by the GitHub Runner. For instance:
+
+```yaml
+apiVersion: actions.summerwind.dev/v1alpha1
+kind: RunnerDeployment
+metadata:
+  name: github-runner
+  namespace: github-system
+spec:
+  replicas: 6
+  template:
+    spec:
+      dockerMTU: 1400
+      repository: $username/$repo
+      env:
+        - name: ARC_DOCKER_MTU_PROPAGATION
+          value: "true"
+```
+
+You can read the discussion regarding this issue in
+(#1406)[https://github.com/actions-runner-controller/actions-runner-controller/issues/1046].
 
 ## Unable to scale to zero with TotalNumberOfQueuedAndInProgressWorkflowRuns