Bug: describe bug here

**Summary**
I am trying to run envoy on ecs fargate with a readonlyRootFileSystem as per ecs best practices. When I run in this mode Envoy crashes because it can't save the envoy bootstrap config. Is it documented anywhere the list of non readonly mountPoints that need to be configured in order to get aws-appmesh-envoy to run with a readonly filesystem?

In have tried adding a mountPoint to /tmp:

{
sourceVolume = "envoy-tmp"
containerPath = "/tmp"
readOnly=false
}

But this still fails.

**Steps to Reproduce**
Run envoy with: readonlyRootFilesystem = true set in the envoy container definition.

**Are you currently working around this issue?**
Not make the envoy container readonly.

Start by reproducing the failure with readonlyRootFilesystem=true in an ECS Fargate Envoy container, including the attempted /tmp mount. Determine which writable mount points are required for the bootstrap configuration, then document those requirements or confirm that this configuration is unsupported.

Written by the indexing model from the issue text.