Add documentation for the EnvFiles Feature Gate

Author: HirazawaUi

content/en/docs/reference/command-line-tools-reference/feature-gates/EnvFiles.md

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+---
+title: EnvFiles
+content_type: feature_gate
+_build:
+  list: never
+  render: false
+
+stages:
+  - stage: alpha
+    defaultValue: false
+    fromVersion: "1.34"
+---
+Support defining container's Environment Variable Values via File.
+See [Define Environment Variable Values Using An Init Container](/docs/tasks/inject-data-application/define-environment-variable-via-file) for more details.

content/en/docs/tasks/inject-data-application/define-environment-variable-via-file.md

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+---
+title: Define Environment Variable Values Using An Init Container
+content_type: task
+min-kubernetes-server-version: v1.34
+weight: 30
+---
+
+<!-- overview -->
+
+{{< feature-state feature_gate_name="EnvFiles" >}}
+
+This page show how to configure environment variables for containers in a Pod via file.
+
+## {{% heading "prerequisites" %}}
+
+{{< include "task-tutorial-prereqs.md" >}}
+
+{{% version-check %}}
+
+<!-- steps -->
+
+## How the design works
+
+In this exercise, you will create a Pod that sources environment variables from files, 
+projecting these values into the running container.
+
+{{% code_sample file="pods/inject/envars-file-container.yaml" %}}
+
+In this manifest, you can see the `initContainer` mounts an `emptyDir` volume and writes environment variables to a file within it,
+and the regular containers reference both the file and the environment variable key 
+through the `fileKeyRef` field without needing to mount the volume. 
+When `optional` field is set to false, the specified `key` in `fileKeyRef` must exist in the environment variables file.
+
+The volume will only be mounted to the container that writes to the file
+(`initContainer`), while the consumer container that consumes the environment variable will not have the volume mounted.
+
+During container initialization, the kubelet retrieves environment variables 
+from specified files in the `emptyDir` volume and exposes them to the container.
+
+{{< note >}}
+All container types (initContainers, regular containers, sidecars containers,
+and ephemeral containers) support environment variable loading from files.
+
+While these environment variables can store sensitive information, 
+`emptyDir` volumes don't provide the same protection mechanisms as
+dedicated Secret objects. Therefore, exposing confidential environment variables 
+to containers through this feature is not considered a security best practice.
+{{< /note >}}
+
+
+Create the Pod:
+
+```shell
+kubectl apply -f https://k8s.io/examples/pods/inject/envars-file-container.yaml
+```
+
+Verify that the container in the Pod is running:
+
+```shell
+# If the new Pod isn't yet healthy, rerun this command a few times.
+kubectl get pods
+```
+
+Check container logs for environment variables:
+
+```shell
+kubectl logs dapi-test-pod -c use-envfile | grep DB_ADDRESS
+```
+
+The output shows the values of selected environment variables:
+
+```
+DB_ADDRESS=address
+```
+
+## `.env` File Syntax
+
+The following syntax rules apply to environment files:
+
+* Blank Lines: Blank lines are ignored.
+
+* Leading Spaces: Leading spaces on all lines are ignored.
+
+* Variable Declaration: Variables must be declared as `VAR=VAL`. Spaces surrounding `=` and trailing spaces are ignored.
+    ```
+    VAR=VAL → VAL
+    ```
+
+* Comments: Lines beginning with # are treated as comments and ignored.
+    ```
+    # comment
+    VAR=VAL → VAL
+
+    VAR=VAL # not a comment → VAL # not a comment
+    ```
+
+* Line Continuation: A backslash (`\`) at the end of a variable declaration line indicates the value continues on the next line. The lines are joined with a single space.
+    ```
+    VAR=VAL \
+    VAL2
+    → VAL VAL2
+    ```
+
+
+
+## {{% heading "whatsnext" %}}
+
+* Learn more about [environment variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information/).
+* Read [Defining Environment Variables for a Container](/docs/tasks/inject-data-application/define-environment-variable-container/)
+* Read [Expose Pod Information to Containers Through Environment Variables](/docs/tasks/inject-data-application/environment-variable-expose-pod-information)

content/en/examples/pods/inject/envars-file-container.yaml

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+apiVersion: v1
+kind: Pod
+metadata:
+  name: envfile-test-pod
+spec:
+  initContainers:
+    - name: setup-envfile
+      image:  nginx
+      command: ['sh', '-c', 'echo "DB_ADDRESS=address\nREST_ENDPOINT=endpoint" > /data/config.env']
+      volumeMounts:
+        - name: config
+          mountPath: /data
+  containers:
+    - name: use-envfile
+      image: nginx
+      command: [ "/bin/sh", "-c", "env" ]
+      env:
+        - name: DB_ADDRESS
+          valueFrom:
+            fileKeyRef:
+              path: config.env
+              volumeName: config
+              key: DB_ADDRESS
+              optional: false
+  restartPolicy: Never
+  volumes:
+    - name: config
+      emptyDir: {}