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Building and Pushing Docker Images

This guide will walk you through building and pushing Docker images with rok8s-scripts. These scripts simplify caching from previously built images.

Initial Project Structure

All rok8s-scripts configuration lives in a deploy directory at the root of your project by default. In this example, we have a simple Python app with a Dockerfile in place. We'll add that deploy directory and add a rok8s-scripts build config file (build.config).

app-dir/
├── deploy/           // Default directory for rok8s-scripts configs and files
│   └── build.config  // rok8s-scripts build configs
├── app.py
├── Dockerfile
├── README.md
└── requirements.txt

This build.config file sets a number of environment variables that the Docker related scripts will read from:

DOCKERFILE=('Dockerfile')
EXTERNAL_REGISTRY_BASE_DOMAIN='012345678911.dkr.ecr.us-east-1.amazonaws.com'
REPOSITORY_NAME='app'

In this example, we'll be pushing our image to an AWS ECR repository. There are additional docs for both AWS and Google Cloud that cover authentication. For the purpose of this example, we'll assume the authentication step has already happened.

Building a Docker Image

The following command runs the rok8s-scripts docker-build script to build a Docker image with the build.config file.

docker-build -f deploy/build.config

By default, this will attempt to pull the following images, and use them as --cache-from flags in the Docker build command:

"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:$PREVIOUS_COMMIT"
"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:$CI_BRANCH"
"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:master"

Pushing a Docker Image

Similarly, once that image has been built, we can push and tag it using the docker-push script:

docker-push -f deploy/build.config

This will attempt to push the image with the following tags:

"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:${CI_SHA1}"
"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:${CI_REF}"
"${EXTERNAL_REGISTRY_BASE_DOMAIN}/${REPOSITORY_NAME}:build_${CI_BUILD_NUM}"

An important note here is that $CI_REF defaults to $CI_BRANCH, enabling caching in the build step.

Change Detection

In some cases it will be beneficial to have an indicator of when a container that was built using the docker-build command actually created a new layer, as opposed to it just using cached layers. There is a feature called ROK8S_ENABLE_CHANGE_DETECTION that can help with this.

When set to true, ROK8S_ENABLE_CHANGE_DETECTION will compare the sha256 of the newly built container with the sha256 of the cached container for that branch. It will output a file called .changesDetected. This file will contain true if there were changes, or false if the container ID is identical to the cache.