* fix panic from nil ReschedulePolicy
commit 279775082c (pr #26279)
intended to return an error for sysbatch jobs with a reschedule block,
but in bypassing populating the `ReschedulePolicy`'s pointer fields,
a nil pointer panic occurred before the job could get rejected
with the intended error.
in particular, in `command/agent/job_endpoint.go`, `func ApiTgToStructsTG`,
```
if taskGroup.ReschedulePolicy != nil {
tg.ReschedulePolicy = &structs.ReschedulePolicy{
Attempts: *taskGroup.ReschedulePolicy.Attempts,
Interval: *taskGroup.ReschedulePolicy.Interval,
```
`*taskGroup.ReschedulePolicy.Interval` was a nil pointer.
* fix e2e test jobs
In #23966 we introduced an official Docker client and did not notice that in
contrast to our previous 3rd party client, the official SDK PullOptions object
expects a base64 encoded JSON with username and password, instead of username/
password pair.
