Support Alpine-based containers with Agent Injection

src/main/java/org/csanchez/jenkins/plugins/kubernetes/PodTemplate.java

@@ -192,6 +192,8 @@ protected static MessageDigest getLabelDigestFunction() {
 
     private boolean agentInjection;
 
+    private String agentInjectionImage;
+
     /**
      * Persisted yaml fragment
      */
@@ -649,6 +651,16 @@ public void setAgentInjection(boolean agentInjection) {
         this.agentInjection = agentInjection;
     }
 
+    @CheckForNull
+    public String getAgentInjectionImage() {
+        return agentInjectionImage;
+    }
+
+    @DataBoundSetter
+    public void setAgentInjectionImage(@CheckForNull String agentInjectionImage) {
+        this.agentInjectionImage = Util.fixEmptyAndTrim(agentInjectionImage);
+    }
+
     public List<TemplateEnvVar> getEnvVars() {
         if (envVars == null) {
             return Collections.emptyList();
@@ -1199,6 +1211,7 @@ public String toString() {
                 + (!unwrapped ? "" : ", unwrapped=" + unwrapped)
                 + (agentContainer == null ? "" : ", agentContainer='" + agentContainer + '\'')
                 + (!agentInjection ? "" : ", agentInjection=" + agentInjection)
+                + (agentInjectionImage == null ? "" : ", agentInjectionImage='" + agentInjectionImage + '\'')
                 + '}';
     }
 }

src/main/java/org/csanchez/jenkins/plugins/kubernetes/PodTemplateBuilder.java

@@ -324,10 +324,18 @@ public Pod build() {
                 .filter(c -> c.getWorkingDir() == null)
                 .forEach(c -> c.setWorkingDir(workingDir));
         String agentImage = DEFAULT_AGENT_IMAGE;
-        if (cloud != null && StringUtils.isNotEmpty(cloud.getJnlpregistry())) {
-            agentImage = Util.ensureEndsWith(cloud.getJnlpregistry(), "/") + agentImage;
-        } else if (StringUtils.isNotEmpty(DEFAULT_JNLP_DOCKER_REGISTRY_PREFIX)) {
-            agentImage = Util.ensureEndsWith(DEFAULT_JNLP_DOCKER_REGISTRY_PREFIX, "/") + agentImage;
+
+        // Priority 1: Template-level custom image (most specific)
+        if (StringUtils.isNotEmpty(template.getAgentInjectionImage())) {
+            agentImage = template.getAgentInjectionImage();
+        } else {
+            // Priority 2: Cloud-level registry prefix
+            if (cloud != null && StringUtils.isNotEmpty(cloud.getJnlpregistry())) {
+                agentImage = Util.ensureEndsWith(cloud.getJnlpregistry(), "/") + agentImage;
+            } else if (StringUtils.isNotEmpty(DEFAULT_JNLP_DOCKER_REGISTRY_PREFIX)) {
+                // Priority 3: System property prefix
+                agentImage = Util.ensureEndsWith(DEFAULT_JNLP_DOCKER_REGISTRY_PREFIX, "/") + agentImage;
+            }
         }
         if (StringUtils.isBlank(agentContainer.getImage())) {
             agentContainer.setImage(agentImage);

src/main/java/org/csanchez/jenkins/plugins/kubernetes/PodTemplateUtils.java

@@ -503,6 +503,7 @@ public static PodTemplate combine(PodTemplate parent, PodTemplate template) {
         podTemplate.setSupplementalGroups(h.resolve(PodTemplate::getSupplementalGroups, Objects::isNull));
         podTemplate.setAgentContainer(h.resolve(PodTemplate::getAgentContainer, PodTemplateUtils::isNullOrEmpty));
         podTemplate.setAgentInjection(h.resolve(PodTemplate::isAgentInjection, v -> !v));
+        podTemplate.setAgentInjectionImage(h.resolve(PodTemplate::getAgentInjectionImage, PodTemplateUtils::isNullOrEmpty));
         if (template.isHostNetworkSet()) {
             podTemplate.setHostNetwork(template.isHostNetwork());
         } else if (parent.isHostNetworkSet()) {

src/main/resources/org/csanchez/jenkins/plugins/kubernetes/PodTemplate/config.jelly

@@ -53,6 +53,10 @@ THE SOFTWARE.
     <f:checkbox/>
   </f:entry>
 
+  <f:entry field="agentInjectionImage" title="${%Agent injection image}">
+    <f:textbox/>
+  </f:entry>
+
   <f:entry field="containers" title="${%Containers}" description="${%List of container in the agent pod}">
       <f:repeatableHeteroProperty field="containers" hasHeader="true" addCaption="${%Add Container}"
                                     deleteCaption="${%Delete Container}" />

src/main/resources/org/csanchez/jenkins/plugins/kubernetes/PodTemplate/help-agentInjectionImage.html

@@ -0,0 +1,29 @@
+<p>
+Specifies the container image to use for agent injection when <strong>Inject Jenkins agent in agent container</strong> is enabled.
+</p>
+
+<p>
+This is useful when your agent container is based on Alpine Linux (musl libc) and needs an Alpine-compatible agent image.
+</p>
+
+<p>
+If not specified, the plugin uses the default agent image (<code>jenkins/inbound-agent:3355.v388858a_47b_33-3-jdk21</code>)
+with any configured registry prefix.
+</p>
+
+<p><strong>Examples:</strong></p>
+<ul>
+  <li><code>jenkins/inbound-agent:3355.v388858a_47b_33-3-alpine-jdk21</code> - Use the official Alpine variant</li>
+  <li><code>my-registry.com/custom-agent:latest</code> - Use a custom registry and image</li>
+</ul>
+
+<p>
+<strong>Important:</strong> The <code>jenkins-agent</code> script requires <code>bash</code>.
+If your agent container is Alpine-based and doesn't include bash, add it to your container:
+</p>
+<pre>RUN apk add --no-cache bash</pre>
+
+<p>
+<strong>Note:</strong> This setting only applies when agent injection is enabled.
+It does not affect the agent container's base image.
+</p>

src/test/java/org/csanchez/jenkins/plugins/kubernetes/PodTemplateBuilderTest.java

@@ -262,6 +262,57 @@ public void testValidateDockerRegistryPrefixOverrideForInitContainer(boolean dir
         assertThat(pod.getMetadata().getLabels(), hasEntry("jenkins", "slave"));
     }
 
+    @Test
+    public void testAgentInjectionWithCustomImage() throws Exception {
+        PodTemplate template = new PodTemplate();
+        template.setAgentInjection(true);
+        template.setAgentInjectionImage("jenkins/inbound-agent:alpine-jdk21");
+        template.setYaml(loadYamlFile("pod-busybox.yaml"));
+        setupStubs();
+
+        Pod pod = new PodTemplateBuilder(template, slave).build();
+
+        assertNotNull(pod.getSpec().getInitContainers());
+        assertEquals(1, pod.getSpec().getInitContainers().size());
+        Container initContainer = pod.getSpec().getInitContainers().get(0);
+        assertEquals("set-up-jenkins-agent", initContainer.getName());
+        assertEquals("jenkins/inbound-agent:alpine-jdk21", initContainer.getImage());
+    }
+
+    @Test
+    public void testAgentInjectionImageOverridesRegistry() throws Exception {
+        cloud.setJnlpregistry("registry.example.com");
+
+        PodTemplate template = new PodTemplate();
+        template.setAgentInjection(true);
+        template.setAgentInjectionImage("custom-registry.io/my-agent:alpine");
+        template.setYaml(loadYamlFile("pod-busybox.yaml"));
+        setupStubs();
+
+        Pod pod = new PodTemplateBuilder(template, slave).build();
+
+        assertNotNull(pod.getSpec().getInitContainers());
+        Container initContainer = pod.getSpec().getInitContainers().get(0);
+        assertEquals("custom-registry.io/my-agent:alpine", initContainer.getImage());
+    }
+
+    @Test
+    public void testAgentInjectionWithoutCustomImageUsesDefault() throws Exception {
+        cloud.setJnlpregistry("registry.example.com");
+
+        PodTemplate template = new PodTemplate();
+        template.setAgentInjection(true);
+        // agentInjectionImage NOT set
+        template.setYaml(loadYamlFile("pod-busybox.yaml"));
+        setupStubs();
+
+        Pod pod = new PodTemplateBuilder(template, slave).build();
+
+        assertNotNull(pod.getSpec().getInitContainers());
+        Container initContainer = pod.getSpec().getInitContainers().get(0);
+        assertEquals("registry.example.com/" + DEFAULT_AGENT_IMAGE, initContainer.getImage());
+    }
+
     @Test
     @Issue("JENKINS-50525")
     public void testBuildWithCustomWorkspaceVolume() throws Exception {