docs: add Kubernetes CKS study notes

kubernetes/level 2/task-4.md

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+# Assignment
+
+The Nautilus DevOps team is working on to setup some pre-requisites for an application that will send the greetings to different users. There is a sample deployment, that needs to be tested. Below is a scenario which needs to be configured on Kubernetes cluster. Please find below more details about it.
+
+
+Create a pod named print-envars-greeting.
+
+Configure spec as, the container name should be print-env-container and use bash image.
+
+Create three environment variables:
+
+a. GREETING and its value should be Welcome to
+
+b. COMPANY and its value should be DevOps
+
+c. GROUP and its value should be Industries
+
+Use command ["/bin/sh", "-c", 'echo "$(GREETING) $(COMPANY) $(GROUP)"'] (please use this exact command), also set its restartPolicy policy to Never to avoid crash loop back.
+
+You can check the output using kubectl logs -f print-envars-greeting command.
+
+
+Note: The kubectl utility on the jump-host has been configured to work with the Kubernetes cluster.
+
+# Solution
+
+# Kubernetes Pod with Env Vars — `print-envars-greeting`
+
+A one-shot pod that echoes three environment variables using Kubernetes' `$(VAR)` substitution.
+Applied inline via a heredoc — no manifest file on disk.
+
+## Apply (heredoc → kubectl)
+
+```bash
+kubectl apply -f - <<'EOF'
+apiVersion: v1
+kind: Pod
+metadata:
+  name: print-envars-greeting
+spec:
+  restartPolicy: Never
+  containers:
+    - name: print-env-container
+      image: bash
+      command: ["/bin/sh", "-c", 'echo "$(GREETING) $(COMPANY) $(GROUP)"']
+      env:
+        - name: GREETING
+          value: "Welcome to"
+        - name: COMPANY
+          value: "DevOps"
+        - name: GROUP
+          value: "Industries"
+EOF
+```
+
+## Check the output
+
+```bash
+kubectl logs -f print-envars-greeting
+```
+
+Expected: `Welcome to DevOps Industries`
+
+## How it works
+
+### The heredoc apply pattern — quoting matters a lot here
+
+- **`kubectl apply -f -`** reads from **stdin**; nothing written to disk.
+- **`<<'EOF'` (delimiter quoted)** is critical for this task. The command contains `$(GREETING)`,
+  `$(COMPANY)`, `$(GROUP)`. If the heredoc delimiter were **unquoted**, the jump-host's shell
+  would interpret `$(...)` as **command substitution** and try to run `GREETING` as a command —
+  mangling the manifest before kubectl ever sees it. Quoting `EOF` passes the `$(...)` through
+  literally so Kubernetes (not the shell) handles the expansion.
+
+### `$(VAR)` is Kubernetes substitution, not shell substitution
+
+The exact command uses `$(GREETING)`, not `${GREETING}` or `$GREETING`. In a container's
+`command`/`args`, **Kubernetes itself** expands `$(VAR)` references using the container's declared
+`env` **before** the process starts. So Kubernetes rewrites the command to
+`echo "Welcome to DevOps Industries"`, and `/bin/sh` then just echoes that literal string.
+
+This is a subtle but important distinction:
+- **`$(VAR)`** — expanded by **Kubernetes** from the `env` list at container start.
+- **`${VAR}` / `$VAR`** — expanded by the **shell** at runtime.
+
+Both would produce the same output here (since `sh` also has the env vars), but the task pins the
+`$(VAR)` form, and it's Kubernetes doing the substitution. A gotcha to know: `$(VAR)` only
+resolves if a matching `env` entry exists; an undefined `$(FOO)` is left **literal** rather than
+erroring.
+
+### The env vars
+
+Three `env` entries — `GREETING="Welcome to"`, `COMPANY="DevOps"`, `GROUP="Industries"` — supply
+the values Kubernetes substitutes into the command. Values are quoted because they contain spaces
+(`Welcome to`).
+
+### `restartPolicy: Never`
+
+The command echoes once and exits `0` — a completed run, not a long-running process. With the
+default `restartPolicy: Always`, Kubernetes would see the container exit and keep restarting it,
+driving `CrashLoopBackOff` (even though it "succeeded"). `Never` tells Kubernetes not to restart
+it, so the pod ends in `Completed` state cleanly. This is why the task specifies it.
+
+### The `bash` image
+
+`image: bash` pulls the official BusyBox-based bash image, which provides `/bin/sh` to run the
+command. It runs the echo and exits — exactly the one-shot behavior wanted.
+
+## Verify
+
+```bash
+# Pod ran to completion
+kubectl get pod print-envars-greeting            # STATUS: Completed
+
+# The greeting output
+kubectl logs print-envars-greeting
+```
+
+Expected — `print-envars-greeting` in `Completed` status, and the logs printing
+`Welcome to DevOps Industries`.
+
+> `Completed` (not `Running`) is correct here — the pod's job was to print once and exit. If it
+> shows `CrashLoopBackOff`, `restartPolicy: Never` didn't take.