Docker is a platform for running applications in containers.
You can specify how to build an image by Writing a Dockerfile file. Once your have your Dockerfile in a GitHub (Or Bitbucket) repository it’s possible to trigger DockerHub to build a Docker image for you automatically so when you update your Dockerfile the Docker image is being updated in DockerHub as well.
The process of doing an automated build is covered here so I’ll just add one point that may not be clear.
When you create an automated build in DockerHub the image is only being built when you update your GitHub repository.
You can trigger a build when going to Dashboard-> Your repository -> Build Settings and then Click on Trigger as shown below
If you go to the Build Details Tab you should see that the build is in Queued status.
It can take couple of minutes until the build is finished (You need to refresh the page to see the new status).