**What this PR does / why we need it**:
The --pod-infra-container-image parameter allows the user to specify
an arbitrary image to be used as the pod infra container (AKA
sandbox), an internal piece of the dockershim implementation of the
Container Runtime Interface.
The dockershim does not have access to any of the pod-level image pull
credentials configuration, so if the user specifies an image from a
private registry, the image pull will fail.
This change allows the dockershim to read local docker configuration
(e.g. /root/.docker/config.json) and use it when pulling the pod infra
container image.
**Which issue this PR fixes**: fixes#45738
**Special notes for your reviewer**:
The changes to fake_client for writing local config files deserve some
attention.
**Release note**:
```release-note
NONE
```
