Building Images with Dockerfile

Overview

Author a clear Dockerfile for a small app: choose a base image, install deps, copy code, set USER, and define CMD/ENTRYPOINT.

A Dockerfile is a build recipe. Each instruction can create a layer — order matters for cache and size (optimisation is Module 6).

This is a core tutorial in Module 5 · Dockerfile of the REBASH Academy Docker for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• Working with Docker Images

Learning Objectives

By the end of this tutorial, you will be able to:

• Use FROM, RUN, COPY, WORKDIR
• Contrast CMD vs ENTRYPOINT
• Set ENV / ARG, EXPOSE, LABEL, USER
• Build and run a local image

Architecture

This topic’s control points and relationships are shown below.

Theory

What

A Dockerfile is a recipe of instructions that BuildKit executes to produce an image. Core instructions include FROM, RUN, COPY, WORKDIR, ENV, ARG, USER, EXPOSE, CMD, and ENTRYPOINT. The build context is the directory you send to the daemon (filtered by .dockerignore).

Why

Hand-built containers are not reproducible. Dockerfiles give teams a reviewable, CI-friendly definition of how production artefacts are made. Instruction choice affects size, cache hit rate, and security (especially USER and what you COPY).

How it works

Each instruction creates a layer (conceptually). FROM selects a base. RUN executes build-time commands. COPY adds files from the context; prefer it over ADD unless you need a specific ADD feature. ARG values are build-time only; ENV persists into the runtime image. CMD and ENTRYPOINT together define the default process — know shell vs exec form. EXPOSE documents ports; it does not publish them. Builds run with BuildKit on modern Docker (DOCKER_BUILDKIT=1).

Instruction	Role
FROM	Base image
RUN	Execute at build time
COPY / ADD	Prefer COPY for files
WORKDIR	Working directory
ENV / ARG	Runtime env vs build args
USER	Drop root when possible
EXPOSE	Document ports (not publish)
CMD / ENTRYPOINT	Default process

Key concepts

• Build context — only send what you need
• Layer caching — order stable steps before frequently changing ones
• Exec form — CMD ["python","app.py"] avoids shell surprises
• Reproducibility — pin base tags or digests

Common pitfalls

• COPY . . without a .dockerignore (secrets, .git, node_modules)
• Running as root in the final image by default
• Confusing ARG (build) with runtime configuration
• Using latest bases that break builds unexpectedly

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-docker/module-05/app && cd ~/rebash-docker/module-05/app

Focus: write a Dockerfile, build a tagged image, and run it

Step 1 – Dockerfile build

cat > app.py << 'EOF'
print("hello from dockerfile lab")
EOF
cat > Dockerfile << 'EOF'
FROM python:3.12-alpine
WORKDIR /app
COPY app.py .
USER nobody
CMD ["python", "app.py"]
EOF
docker build -t rebash-df:lab .
docker run --rm rebash-df:lab

Step 2 – Inspect image and cleanup

docker image inspect rebash-df:lab --format '{{.Config.User}}'
docker rmi rebash-df:lab

Final step – Cleanup note

docker rmi rebash-df:lab 2>/dev/null || true
# Keep ~/rebash-docker/ for later tutorials

Validation

• Lab commands run under ~/rebash-docker/module-05/app/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Building Images with Dockerfile always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for docker as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

COPY . . without a .dockerignore (secrets, .git, node_modules)

Validate assumptions against the Theory section and official docs before changing production.

Running as root in the final image by default

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Building Images with Dockerfile changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Building Images with Dockerfile is essential for Cloud and DevOps engineers working with docker. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. What do FROM/COPY/RUN/CMD/ENTRYPOINT each do?
2. Build context is huge — how do you shrink it?
3. Why avoid running as root in the final image?
4. Difference between CMD and ENTRYPOINT?
5. How do build args differ from runtime env?

Sample answer — question 2

Read the Dockerfile and docker history; rebuild with --progress=plain to see failing RUN lines.

Sample answer — question 4

Do not COPY secrets into layers. Use multi-stage builds and non-root users.

Related Tutorials

• Course overview
• Dockerfile Best Practices and Multi-Stage Builds

References

• Dockerfile reference