How to build a Docker image on Linux

Whether you’re pushing to dockerhub, or building an image for local use, the process for building docker images is the same.

Building the Docker image

First, let’s create a directory for us to create a dockerfile:

sudo mkdir /sample-docker && cd /sample-docker 

Once complete, we can create our dockerfile:

sudo touch Dockerfile

Place the contents of your dockerfile into the Dockerfile file that you just created:

cat app.dockerfile > /sample-docker/Dockerfile

In this case, app.dockerfile is the original file that we were building the dockerfile in. You can place the contents of your dockerfile directly into the Dockerfile that we just created also. For example purposes, a sample dockerfile is below:

# Original app.dockerfile
# This was copied to /sample-docker/dockerfile

# Take the official Postgres image and add a username, password, and listen port
FROM postgres
ENV POSTGRES_PASSWORD postgres
ENV POSTGRES_PORT 5432
USER postgres

From there, we can build the image:

docker build -t postgres-updated .

In this case, “postgres-updated” is the name of the image. However, this can be changed to anything you like. You will need to be in the directory where your Dockerfile is located for the command above to work.

Once the image has finished building, we can take this image:

docker build -t postgres-updated .

Pushing to Dockerhub

Once the image has been built, we can tag it and upload it to dockerhub. We can tag the image as below:

docker tag postgres-updated <reponame>/postgres-updated

Where is the name of our docker repository. Once tagged, we can login to our docker account via the command line:

docker login -u "username" -p "password" docker.io

Where username is your dockerhub username, and password is your dockerhub password.

Lastly, we can go ahead and push the image to our docker repository:

docker push <reponame>/postgres-updated:latest

Where is the name of your docker repository.