[SRVKS-892]: Add init container docs

modules/serverless-admin-init-containers.adoc

@@ -0,0 +1,44 @@
+// Module included in the following assemblies:
+//
+// * /serverless/admin_guide/serverless-configuration.adoc
+
+:_content-type: PROCEDURE
+[id="serverless-admin-init-containers_{context}"]
+= Enabling init containers
+
+link:https://kubernetes.io/docs/concepts/workloads/pods/init-containers/[Init containers] are specialized containers that are run before application containers in a pod. They are generally used to implement initialization logic for an application, which may include running setup scripts or downloading required configurations. You can enable the use of init containers for Knative services by modifying the `KnativeServing` custom resource (CR).
+
+[NOTE]
+====
+Init containers may cause longer application start-up times and should be used with caution for serverless applications, which are expected to scale up and down frequently.
+====
+
+.Prerequisites
+
+* You have installed {ServerlessOperatorName} and Knative Serving on your cluster.
+
+ifdef::openshift-enterprise[]
+* You have cluster administrator permissions.
+endif::[]
+
+ifdef::openshift-dedicated[]
+* You have cluster or dedicated administrator permissions.
+endif::[]
+
+.Procedure
+
+* Enable the use of init containers by adding the `kubernetes.podspec-init-containers` flag to the `KnativeServing` CR:
++
+.Example KnativeServing CR
+[source,yaml]
+----
+apiVersion: operator.knative.dev/v1alpha1
+kind: KnativeServing
+metadata:
+  name: knative-serving
+spec:
+  config:
+    features:
+      kubernetes.podspec-init-containers: enabled
+...
+----

modules/serverless-config-emptydir.adoc

@@ -9,14 +9,18 @@
 
 `emptyDir` volumes are empty volumes that are created when a pod is created, and are used to provide temporary working disk space. `emptyDir` volumes are deleted when the pod they were created for is deleted.
 
-The `"kubernetes.podspec-volumes-emptydir"` extension controls whether `emptyDir` volumes can be used with Knative Serving. To enable using `emptyDir` volumes, you must modify the `KnativeServing` custom resource (CR) to include the following YAML:
+The `kubernetes.podspec-volumes-emptydir` extension controls whether `emptyDir` volumes can be used with Knative Serving. To enable using `emptyDir` volumes, you must modify the `KnativeServing` custom resource (CR) to include the following YAML:
 
+.Example KnativeServing CR
 [source,yaml]
 ----
-...
+apiVersion: operator.knative.dev/v1alpha1
+kind: KnativeServing
+metadata:
+  name: knative-serving
 spec:
   config:
     features:
-      "kubernetes.podspec-volumes-emptydir": enabled
+      kubernetes.podspec-volumes-emptydir: enabled
 ...
 ----

modules/serverless-init-containers-apps.adoc

@@ -0,0 +1,47 @@
+// Module included in the following assemblies:
+//
+// * /serverless/develop/serverless-applications.adoc
+
+:_content-type: PROCEDURE
+[id="serverless-init-containers-apps_{context}"]
+= Configuring init containers
+
+link:https://kubernetes.io/docs/concepts/workloads/pods/init-containers/[Init containers] are specialized containers that are run before application containers in a pod. They are generally used to implement initialization logic for an application, which may include running setup scripts or downloading required configurations.
+
+[NOTE]
+====
+Init containers may cause longer application start-up times and should be used with caution for serverless applications, which are expected to scale up and down frequently.
+====
+
+Multiple init containers are supported in a single Knative service spec. Knative provides a default, configurable naming template if a template name is not provided. The init containers template can be set by adding an appropriate value in a Knative `Service` object spec.
+
+.Prerequisites
+
+* {ServerlessOperatorName} and Knative Serving are installed on your cluster.
+
+* Before you can use init containers for Knative services, an administrator must add the `kubernetes.podspec-init-containers` flag to the `KnativeServing` custom resource (CR). See the {ServerlessProductName} "Global configuration" documentation for more information.
+
+.Procedure
+
+* Add the `initContainers` spec to a Knative `Service` object:
++
+.Example service spec
+[source,yaml]
+----
+apiVersion: serving.knative.dev/v1
+kind: Service
+...
+spec:
+  template:
+    spec:
+      initContainers:
+        - imagePullPolicy: IfNotPresent <1>
+          image: <image_uri> <2>
+          volumeMounts: <3>
+            - name: data
+              mountPath: /data
+...
+----
+<1> The link:https://kubernetes.io/docs/concepts/containers/images/#image-pull-policy[image pull policy] when the image is downloaded.
+<2> The URI for the init container image.
+<3> The location where volumes are mounted within the container file system.

serverless/admin_guide/serverless-configuration.adoc

@@ -16,7 +16,9 @@ The `spec.config` in the Knative custom resources have one `<name>` entry for ea
 include::modules/serverless-channel-default.adoc[leveloffset=+1]
 
 // Knative Serving
+// deployments
 include::modules/knative-serving-CR-system-deployments.adoc[leveloffset=+1]
+// enable emptydirs
 include::modules/serverless-config-emptydir.adoc[leveloffset=+1]
 // global https redirect
 include::modules/serverless-https-redirect-global.adoc[leveloffset=+1]
@@ -26,6 +28,8 @@ include::modules/serverless-url-scheme-external-routes.adoc[leveloffset=+1]
 include::modules/serverless-kourier-gateway-service-type.adoc[leveloffset=+1]
 // Enabling PVC for Serving
 include::modules/serverless-enabling-pvc-support.adoc[leveloffset=+1]
+// enable init containers
+include::modules/serverless-admin-init-containers.adoc[leveloffset=+1]
 
 ifdef::openshift-enterprise[]
 [id="additional-resources_knative-serving-CR-config"]

serverless/develop/serverless-applications.adoc

@@ -57,11 +57,12 @@ ifdef::openshift-enterprise[]
 // Using Knative services w/ restrictive NetworkPolicies
 include::modules/serverless-services-network-policies.adoc[leveloffset=+1]
 endif::[]
-// should maybe move to admin guide? asked serving team
+// move to admin guide, outside scope of this PR
 
+// config init containers
+include::modules/serverless-init-containers-apps.adoc[leveloffset=+1]
 // HTTPS redirection
 include::modules/serverless-https-redirect-service.adoc[leveloffset=+1]
-// asked serving team why a user would need this for abstract rewrite, waiting for details
 
 [id="additional-resources_serverless-applications"]
 [role="_additional-resources"]