diff --git a/listings/exercises/ex12a-containerfile-steps/Containerfile b/listings/exercises/ex12a-containerfile-steps/Containerfile
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+++ b/listings/exercises/ex12a-containerfile-steps/Containerfile
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+FROM debian:12
+
+RUN apt-get update && apt-get install -y curl
+
+WORKDIR /app
+
+COPY hello.sh .
+RUN chmod +x hello.sh
+
+ENV NAME=Trainee
+
+CMD ["./hello.sh"]
diff --git a/listings/exercises/ex12a-containerfile-steps/commands.txt b/listings/exercises/ex12a-containerfile-steps/commands.txt
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+# Exercise 12a — Build a Containerfile step by step
+#
+# Start with an empty Containerfile and add one instruction at a time.
+# After each step: podman build -t myapp:latest .
+# Then inspect the result before moving on.
+
+cd listings/exercises/ex12a-containerfile-steps/
+
+# -----------------------------------------------------------------------
+# Step 1 — Base image
+# -----------------------------------------------------------------------
+# Containerfile:
+#   FROM debian:12
+
+podman build -t myapp:latest .
+podman images myapp:latest          # ~121 MB
+
+# -----------------------------------------------------------------------
+# Step 2 — Install software (creates a new layer)
+# -----------------------------------------------------------------------
+# Add to Containerfile:
+#   RUN apt-get update && apt-get install -y curl
+
+podman build -t myapp:latest .
+podman images myapp:latest          # ~159 MB — layer is visible
+
+# -----------------------------------------------------------------------
+# Step 3 — Working directory
+# -----------------------------------------------------------------------
+# Add to Containerfile:
+#   WORKDIR /app
+
+podman build -t myapp:latest .
+podman run --rm myapp:latest pwd    # prints /app
+
+# -----------------------------------------------------------------------
+# Step 4 — Copy your script into the image
+# -----------------------------------------------------------------------
+# Add to Containerfile:
+#   COPY hello.sh .
+#   RUN chmod +x hello.sh
+
+podman build -t myapp:latest .
+podman run --rm myapp:latest ls -la # hello.sh is there
+
+# -----------------------------------------------------------------------
+# Step 5 — Environment variable
+# -----------------------------------------------------------------------
+# Add to Containerfile:
+#   ENV NAME=Trainee
+
+podman build -t myapp:latest .
+podman run --rm myapp:latest env | grep NAME
+
+# -----------------------------------------------------------------------
+# Step 6 — Start command
+# -----------------------------------------------------------------------
+# Add to Containerfile:
+#   CMD ["./hello.sh"]
+
+podman build -t myapp:latest .
+podman run --rm myapp:latest                 # uses ENV NAME=Trainee
+podman run --rm -e NAME=Frank myapp:latest   # override at runtime
+
+# -----------------------------------------------------------------------
+# Bonus: layers — chained vs. separate RUN
+# -----------------------------------------------------------------------
+# Two separate RUN = two layers:
+#   RUN apt-get update
+#   RUN apt-get install -y curl
+#
+# One chained RUN = one layer:
+#   RUN apt-get update && apt-get install -y curl
+
+podman inspect myapp:latest --format '{{len .RootFS.Layers}} layers'
diff --git a/listings/exercises/ex12a-containerfile-steps/hello.sh b/listings/exercises/ex12a-containerfile-steps/hello.sh
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+++ b/listings/exercises/ex12a-containerfile-steps/hello.sh
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+#!/bin/bash
+echo "Hello, ${NAME:-World}!"
+echo "Hostname: $(hostname)"
+echo "Date: $(date)"
+curl --version | head -1