Support Alpine-based containers with Agent Injection

[zhan9san]

fix #2822

Testing done

Add agentInjectionImage configuration for Alpine support

Adds an optional agentInjectionImage field to PodTemplate, allowing users to specify a custom agent injection image (e.g., Alpine variants) instead
of always using the default glibc-based image.

MOTIVATION
Agent injection currently always uses jenkins/inbound-agent:3355.v388858a_47b_33-3-jdk21, which is incompatible with Alpine containers (musl libc
vs glibc).

CHANGES

• PodTemplate.java: Added agentInjectionImage field with getter/setter
• PodTemplateBuilder.java: Image resolution priority: custom image > registry prefix > system property > default
• PodTemplateUtils.java: Added field to template inheritance
• config.jelly: Added UI textbox field
• help-agentInjectionImage.html: Added documentation

TESTING
Automated: Added 3 test cases in PodTemplateBuilderTest covering custom image usage, priority override, and backward compatibility. All 17 tests
pass.

Manual: Deployed plugin, configured PodTemplate with agentInjectionImage=jenkins/inbound-agent:3355.v388858a_47b_33-3-alpine-jdk21, verified init
container uses specified image, confirmed agent connects with Alpine container.

BACKWARD COMPATIBILITY
Fully compatible - existing templates without the field use default behavior unchanged.

here is the generated template.

  initContainers:
  - command:
    - "sh"
    - "-c"
    - "ls -al"
    image: "busybox:1.36"
    name: "volume-mount-permission"
    securityContext:
      runAsUser: 0
  - command:
    - "/bin/sh"
    - "-c"
    - "cp $(command -v jenkins-agent) /jenkins-agent/jenkins-agent;cp -R /usr/share/jenkins/.\
      \ /jenkins-agent"
    image: "jenkins/inbound-agent:3355.v388858a_47b_33-3-alpine3.23-jdk21"
    name: "set-up-jenkins-agent"
    volumeMounts:
    - mountPath: "/jenkins-agent"
      name: "jenkins-agent"
  nodeSelector:
    kubernetes.io/os: "linux"
  restartPolicy: "Never"
  volumes:
  - emptyDir:
      medium: ""
    name: "workspace-volume"
  - emptyDir: {}
    name: "jenkins-agent"

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