allow image-builder-mk3 to have multiple pull secrets

[mrsimonemms]

For a full explanation of the problem, see this comment in 10298

The image-builder-mk3 requires support for multiple pull secrets to maintain parity with the imagePullSecrets in Kubernetes's pod spec.

Suggested approach

• add in pullSecrets as an array of strings (can remove pullSecret in a future PR, but out of scope for this ticket)
• usage of these pullSecrets then becomes "loop through the secrets, only failing when all have failed"

[atduarte]

@kylos101 how much effort would you say this is?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.