Move topic to Injecting Data section. (#3622)

* Move topic to Injecting Data section.

* Move included file.

docs/tasks/inject-data-application/define-environment-variable-container.md

@@ -0,0 +1,80 @@
+---
+title: Defining Environment Variables for a Container
+redirect_from:
+- "/docs/tasks/configure-pod-container/define-environment-variable-container/"
+- "/docs/tasks/configure-pod-container/define-environment-variable-container.html"
+---
+
+{% capture overview %}
+
+This page shows how to define environment variables when you run a container
+in a Kubernetes Pod.
+
+{% endcapture %}
+
+
+{% capture prerequisites %}
+
+{% include task-tutorial-prereqs.md %}
+
+{% endcapture %}
+
+
+{% capture steps %}
+
+## Defining an environment variable for a container
+
+When you create a Pod, you can set environment variables for the containers
+that run in the Pod. To set environment variables, include the `env` or
+`envFrom` field in the configuration file.
+
+In this exercise, you create a Pod that runs one container. The configuration
+file for the Pod defines an environment variable with name `DEMO_GREETING` and
+value `"Hello from the environment"`. Here is the configuration file for the
+Pod:
+
+{% include code.html language="yaml" file="envars.yaml" ghlink="/docs/tasks/inject-data-application/envars.yaml" %}
+
+1. Create a Pod based on the YAML configuration file:
+
+        kubectl create -f http://k8s.io/docs/tasks/configure-pod-container/envars.yaml
+
+1. List the running Pods:
+
+        kubectl get pods -l purpose=demonstrate-envars
+
+    The output is similar to this:
+
+        NAME            READY     STATUS    RESTARTS   AGE
+        envar-demo      1/1       Running   0          9s
+
+1. Get a shell to the container running in your Pod:
+
+        kubectl exec -it envar-demo -- /bin/bash
+
+1. In your shell, run the `printenv` command to list the environment variables.
+
+        root@envar-demo:/# printenv
+
+    The output is similar to this:
+
+        NODE_VERSION=4.4.2
+        EXAMPLE_SERVICE_PORT_8080_TCP_ADDR=10.3.245.237
+        HOSTNAME=envar-demo
+        ...
+        DEMO_GREETING=Hello from the environment
+
+1. To exit the shell, enter `exit`.
+
+{% endcapture %}
+
+{% capture whatsnext %}
+
+* Learn more about [environment variables](/docs/tasks/configure-pod-container/environment-variable-expose-pod-information/).
+* Learn about [using secrets as environment variables](/docs/user-guide/secrets/#using-secrets-as-environment-variables).
+* See [EnvVarSource](/docs/api-reference/v1.6/#envvarsource-v1-core).
+
+{% endcapture %}
+
+
+{% include templates/task.md %}