feat(root04): author Containers & Docker

Adds all 8 ROOT 04 modules (why containers exist, images & containers, Docker CLI, container networking, container storage, Dockerfile & image build, Docker Compose, containers → Kubernetes) — 43 activities, using the nested Docker Engine Phase 6D built and tested. Modules 1, 2, and 8 are conceptual (no lab by design) — the honest bridge module (M8) explicitly names what Kubernetes adds beyond what this course already taught, without re-teaching Kubernetes itself. Adds 5 new hands-on Docker scenarios plus 2 MCQs, all against the real nested Docker Engine: docker-cli-basics (run/exec/inspect a real container), docker-container-networking (a genuine "works internally, unreachable from host" port-publishing failure, diagnosed and fixed by recreating the container with the correct `-p` flag), docker-container-storage (a real destroy/recreate cycle proving named-volume persistence, not just described), docker-build-image (a real Dockerfile, a real build, a real custom image run), and docker-compose-multiservice (a real two-service Compose application with a real published port). All tested end-to-end via the real scenario-validation API, including negative tests. Wires rootnode-containers-assessment (2 required + 4 random-draw scenarios) using the existing Phase 3B assessment engine. This completes the entire ROOT_ Foundations sequence (ROOT 01-04) — see docs/PHASE6E_ROOT_FOUNDATIONS_COMPLETION.md. Co-Authored-By: Claude Sonnet 5 <[email protected]>
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+# Knowledge Check: Container Networking
+
+---
+
+**1. A container works fine when tested with `docker exec <container> curl localhost:80`, but
+`curl http://localhost:8080/` from the host fails. What's the most likely cause?**
+
+*Answer:* The container was never started with a port-publish flag (`-p 8080:80`) mapping the
+host port to the container's port — the app works internally, but nothing connects the host to
+it.
+
+---
+
+**2. Can you add port publishing to an already-running container?**
+
+*Answer:* No — port publishing is set at container creation time; you must remove and recreate
+the container with the correct `-p` flag.
+
+---
+
+**3. How can one container reach another by name instead of IP?**
+
+*Answer:* If both containers are on the same user-defined Docker network, Docker's built-in DNS
+resolves the target container's NAME automatically — no IP address needed.
+
+---
+
+**4. Why does container-name resolution fail between two containers that AREN'T on the same
+Docker network?**
+
+*Answer:* Docker's per-network DNS only knows about containers actually attached to that
+specific network — being on a different (or no) network means there's no DNS entry for the
+target container at all.
+
+## Instructor Notes
+
+**Facilitation:** Question 1 directly previews this module's Lab — a good moment to build
+anticipation for it.
+
+**Approximate timing:** 5 minutes.