Is it possible to add environment variables in automated builds in docker hub?

Question

I want to automate my build process and need to pass an environment variable to run some of the commands in the Dockerfile. I was wondering if there was any way to do this in Dockerhub. I know docker cloud has something like this, but I was wondering whether the functionality was there in Dockerhub since there is the --build-args argument in the cli for normal building.

Answer 1

Set up Automated builds

Docker Hub (https://hub.docker.com) can automatically build images from source code in an external repository and automatically push the built image to your Docker repositories which will be hosted under your Docker Hub repositories account Eg: https://cloud.docker.com/u/binbash/repository/list

When you set up automated builds (also called autobuilds), you create a list of branches and tags that you want to build into Docker images. When you push code to a source code branch (currently only GitHub / Bitbucket are supported) for one of those listed image tags, the push uses a webhook to trigger a new build, which produces a Docker image. The built image is then pushed to the Docker Hub registry. For detailed implementation steps please refer to https://docs.docker.com/docker-hub/builds/

Environment variables for builds

You can set the values for environment variables (actually they are mapped to build ARG values - docker build --build-arg - to be exclusively used at build-time - https://docs.docker.com/engine/reference/commandline/build/#set-build-time-variables---build-arg).

NOT to be confused with the environment values, ENV VARS, used by your service at runtime (docker run --env MYVAR1=foo - https://docs.docker.com/v17.12/edge/engine/reference/commandline/run/#set-environment-variables--e-env-env-file)

These Environment Variables configured from the Docker Hub UI are used in your build processes when you configure an automated build. Add your build environment variables by clicking the plus sign next to the Build environment variables section, and then entering a variable name and the value.

When you set variable values from the Docker Hub UI, they can be used by the commands you set in hooks files (THIS IS VERY IMPORTANT and will be extended below), but they are stored so that only users who have admin access to the Docker Hub repository can see their values. This means you can use them to safely store access tokens or other information that should remain secret.

Build hook examples (to implement Docker Hub UI Env vars)

Adding variables from the auto-build’s web UI makes them available inside the hooks. In the hook, you’ll have to use that value to set a custom build arg using --build-arg. Finally, you have to use this custom build arg inside your Dockerfile to manually set an environment variable using ENV command or export.

Example:

Say your want an environment variable TERRAFORM_VERSION='0.12.0-beta2' in your build environment

Step 1. Add this in the auto-build’s web UI for ‘build environment variables’

Step 2. Create a custom build hook i.e create a folder called hooks in the same directory as your Dockerfile. Inside the hooks folder, create a file called build. This creates the custom build hook. Docker will use this to build your image. Contents of build:

#!/bin/bash

docker build -t $IMAGE_NAME --build-arg TERRAFORM_VERSION=$TERRAFORM_VERSION .

NOTE: Here $TERRAFORM_VERSION is coming from the web UI.

Step3: In your Dockerfile

ARG TERRAFORM_VERSION
ENV TERRAFORM_VERSION $TERRAFORM_VERSION

NOTE: Here $TERRAFORM_VERSION is coming from the custom build args in your bash script file named build.

Complete example: https://github.com/binbashar/public-docker-images/tree/master/terraform-resources

That's it! It should work now. Probably renaming ‘build environment variables’ to ‘custom hook environment variables’ in Docker Hub will ease the understanding of this concept in the official documentation (https://docs.docker.com/docker-hub/builds/advanced/).

Extra Points!

There are a number of key environment arguments set upon launching a build script, all of which you can use in your hooks and which can all be useful in making custom build-args.

SOURCE_BRANCH: the name of the branch or the tag that is currently being tested.
SOURCE_COMMIT: the SHA1 hash of the commit being tested.
COMMIT_MSG: the message from the commit being tested and built.
DOCKER_REPO: the name of the Docker repository being built.
DOCKERFILE_PATH: the Dockerfile currently being built.
DOCKER_TAG: the Docker repository tag being built.
IMAGE_NAME: the name and tag of the Docker repository being built. (This variable is a combination of DOCKER_REPO:DOCKER_TAG.)