ALL environment variables available in the Docker Hub build environment

The Docker Hub documentation lists all these environment variables, which are available in your scripts in that environment:

    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.)

However, there are quite a few more, which might be useful in a certain environment; these were discovered by just printing the environment variables via a Perl script (which, BTW, you can use in Docker Hub for your own purposes).

• SOURCE_TYPE: this one seems to contain git. It might be different if it's pushed directly, or via the API.
• PATH: is the regular system PATH. It contains /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin.
• MAX_LOG_SIZE: it contains 67108864. Not clear if it can be changed.
• BUILD_CODE: no idea what this is. It contains a code.
• PUSH: it contains true if it's been created via PUSH. Probably other values if it's been automatically launched.
• GIT_TAG and GIT_SHA1: no idea what the first one is, the second is the commit SHA1.
• HOSTNAME: the host it's using to build.
• DOCKER_HOST: the socket the client is using to connect, same as in the local docker installation.
• There's something called SIGNED_URLS, which holds Amazon urls. No idea what this is, or how it can be useful.
• DOCKER_TAG and CACHE_TAG, the tag that's being built, like latest.
• PYTHONUNBUFFERED is equal to 1.
• SOURCE_REPOSITORY_URL the repository it's being built from.
• LOG_BUILD_STEPS is set to FALSE
• DOCKER_CFG contains information on the user, including user tokens.

Out of all these, you can probably use PUSH or even GIT_SHA1 to introspect what's being built, and maybe take some actions. For instance, along with information on the last build you could create a script that built only those Dockerfiles that have changed.