CI suite "Build and push mock-worker Docker image" step fails on dockerhub login

[fastjames]

Describe the bug
When a pull request is created, the CI suite always fails on the [Build and push mock-worker Docker image](https://github.com/runpod/runpod-python/actions/runs/9365387848/job/25780495905?pr=319#logs) step.

To Reproduce
Run the CI suite for an open PR.

Expected behavior
As part of the failing step, the runner should have the credentials needed to authenticate to Docker Hub.

Screenshots
(see link above for failing CI step)

Additional context
The last time this CI step succeeded was on or about 2024-05-10, as seen in #312 .