Use configured registry for jenkins-agent when running in agent-injection mode

[iandrewt]

I was previously using an ECR Pull Through Cache for the inbound-agent image, but this wasn't respected when using agentInjection: true. This PR fixes that

Testing done

Added a unit test, and tested on my own Jenkins instance - expected behaviour present

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[iandrewt]

The agent injection feature is great btw - it lets me keep a maintained runner image without coupling it with a specific version of Jenkins (or even Jenkins itself). But given the jar file itself is only 1.4MB (compared to the compressed 130MB+ of the inbound-agent image), perhaps it would make sense to use a BusyBox image to copy the file from the controller instance instead at https://jenkins-host/jnlpJars/agent.jar. There'd still be the startup script to account for though

[Vlatombe]

Good catch ! Thanks