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Docker Images, Dockerfiles, and Building: Common Errors and Fixes

Last updated on Sep 08, 2026

Docker Images, Dockerfiles, and Building: Common Errors and Fixes

The Create Docker Image using Dockerfile and Dockerfile exercises teach how to build custom Docker images, write Dockerfiles, and manage image tags. These exercises appear across 4 courses. This article covers the most common build errors and Dockerfile pitfalls.

Table of Contents

  1. Missing Dot in docker build
  2. apt install Fails: No apt update
  3. Interactive Prompt Aborts the Build
  4. Multiple CMD Instructions
  5. ENTRYPOINT vs CMD
  6. Container Exits After Starting a Service
  7. Tagging and Removing Images
  8. Common Questions

Missing Dot in docker build

The Build Context

The dot (.) at the end of docker build specifies the build context — the directory whose contents are sent to the Docker daemon:

# WRONG - missing context:
docker build -t myimage:1.0

# CORRECT:
docker build -t myimage:1.0 .

Without the dot, Docker doesn't know where to find the Dockerfile and source files.

apt install Fails: No apt update

Package Not Found on First Install

If your Dockerfile runs apt install nginx without first updating the package index, the build fails:

E: Unable to locate package nginx

Fix: Always run apt update before apt install:

RUN apt update && apt install -y nginx

Interactive Prompt Aborts the Build

apt install Waits for User Confirmation

Even with apt update, apt install nginx without -y will prompt for confirmation, which Docker can't answer:

Do you want to continue? [Y/n] Abort.

Fix: Add -y to auto-confirm:

RUN apt update && apt install -y nginx

Progressive Debugging Pattern

The exercise teaches a progressive debugging approach:

  1. First attempt: RUN apt install nginx → FAILS (no apt update)
  2. Second attempt: RUN apt update && apt install nginx → FAILS (interactive prompt)
  3. Final fix: RUN apt update && apt install -y nginx → SUCCEEDS

Multiple CMD Instructions

Only the Last CMD Survives

If a Dockerfile has multiple CMD instructions, only the last one takes effect:

CMD ["/bin/bash", "-c", "service nginx start"]
CMD ["sleep infinity"]

The first CMD is silently ignored. If the second CMD is wrong, the container will fail:

exec: ";sleep infinity": executable file not found in $PATH

Fix: Combine into a single CMD:

CMD ["/bin/bash", "-c", "service nginx start; sleep infinity"]

ENTRYPOINT vs CMD

ENTRYPOINT Sets the Command, CMD Provides Arguments

Instruction Purpose Override Behavior
ENTRYPOINT Sets the main command Hard to override — requires --entrypoint
CMD Provides default arguments Easily overridden by CLI args

Example:

ENTRYPOINT ["/bin/bash", "-c"]
CMD ["service nginx start"]

Running docker run myimage executes service nginx start. Running docker run myimage sleep 10 overrides CMD to sleep 10.

Key concept: Use ENTRYPOINT for the command that shouldn't change, and CMD for defaults that can be overridden.

Overriding ENTRYPOINT

To override the entrypoint, use --entrypoint:

docker run --name webserver -it --entrypoint /bin/bash django.nv:1.0

This is useful when you need to debug a container or run a different command than what the image was designed for.

Container Exits After Starting a Service

service nginx start Doesn't Keep the Container Alive

CMD ["/bin/bash", "-c", "service nginx start"]

This starts nginx in the background, but the CMD process (bash running the service command) exits immediately, causing the container to stop:

$ docker ps -a
CONTAINER ID   IMAGE       STATUS
abc123         custom-nginx    Exited (0) 3 seconds ago

Fix: Keep the foreground process alive:

CMD ["/bin/bash", "-c", "service nginx start; sleep infinity"]

Tagging and Removing Images <a name="tag-remove>

Renaming an Image with docker tag

docker tag django.nv:1.0 django.nv:1.1

This creates a new tag pointing to the same image. The original django.nv:1.0 still exists.

Removing an Image

docker rmi django.nv:1.0

Common error: If you're inside a Docker container (e.g., after docker run -it), docker command may not be available:

bash: docker: command not found

Fix: Exit the container first with exit, then run docker rmi from the host shell.

Common Questions

Question Answer
My docker build fails with "Unable to locate package." Add apt update && before apt install.
My build hangs at "Do you want to continue?" Add -y flag: apt install -y <package>.
My container exits immediately after starting a service. Add sleep infinity or another foreground process to keep the container alive.
I have multiple CMD lines in my Dockerfile. Only the last CMD takes effect. Combine them into one.
docker rmi says "docker: command not found." You're inside a container. Exit first, then run the command on the host.
What's the difference between ENTRYPOINT and CMD? ENTRYPOINT sets the main command (hard to override). CMD provides default arguments (easily overridden).

Wrap-Up

Dockerfile authoring follows a few key patterns that prevent most build failures:

  • Always run apt update && apt install -y (the -y is critical).
  • Don't use multiple CMD instructions — only the last one matters.
  • Keep the container alive with a foreground process (sleep infinity, nginx -g 'daemon off;', etc.).
  • The dot (.) in docker build is the build context — don't forget it.
  • ENTRYPOINT + CMD pattern: ENTRYPOINT for the fixed command, CMD for overridable defaults.

The DevSecOps Box is stateless — all Docker images and containers are lost on page refresh.