Images

An image is a read-only template that contains all the necessary files, libraries, and configuration required to initialize a container. It acts as a blueprint which defines the application, but is not the actual running process.

Ubuntu Image Examples

To illustrate this, we first start with a Docker image that leverages an existing Ubuntu base image, which contains all the same shared libraries and utilities found in the Ubuntu OS. Similar to how application code can leverage shared libraries, developers can reference existing base images to simplify what dependencies are needed.

docker build . -t techsquawks/ubuntu

docker run -t techsquawks/ubuntu

# Use Ubuntu as base image for container
FROM ubuntu:23.04

# Install latest packages
RUN apt-get update && apt-get install -y locales && rm -rf /var/lib/apt/lists/* \
    && localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
ENV LANG en_US.utf8


# Copy local text file into container
COPY message.txt message.txt

# Print message from file
ENTRYPOINT [ "cat", "message.txt" ]

ubuntu image example

Scratch Base Image

If we don’t want to leverage an existing image, we can leverage the scartch base image, which is the most minimal base image Docker provides, containing no directories or files. This can be useful when wanting to minimize the container and image size or for creating custom base images.

docker build . -t techsquawks/ubuntu

docker run -t techsquawks/ubuntu

# A 'scratch' image is an empty base image.
FROM scratch

# Copy binary executable
COPY hello hello

# Run executable
ENTRYPOINT [ "./hello" ]

ubuntu image example

In the above, we copy the hello binary executable into the image, which has been compiled for the Docker scratch image architecture. We can then execute the binary via the entrypoint.

The COPY command copies the local hello binary executable into the image file system the image file system, so it can be accessed as the container runs. The entrypoint indicates the executable that should be run when running our container.

For a compressive list of Docker commands that can be used when building an image, please refer to the table below: