docker_build.sh

#!/bin/bash -e

# Fast Docker builds on CircleCI
# See this blog post: https://www.tresegnie.com/docker-circleci-speed-cost/

export CONTEXT_FOLDER=???
export IMAGE_NAME=???
export COMMIT_TAG=${CIRCLE_SHA1}
export BRANCH_TAG=${CIRCLE_BRANCH}

# Calculate a content hash of the docker context to avoid pulling/building
# if a docker image with the same context and dockerfile exist on the registry
#
# To test locally on mac, install sha1sum
#   brew install md5sha1sum
export CONTENT_HASH=$(\
	find domains/${DOMAIN_NAME}/services/${SERVICE_NAME} \( -type l -o -type f \) -print0 \
    | sort -z \
    | xargs -0 sha1sum \
    | sha1sum \
    | awk '{ print $1 }' \
)

if ! docker manifest inspect ${IMAGE_NAME}:${CONTENT_HASH}
then
    docker pull ${IMAGE_NAME}:${COMMIT_TAG} \
        || docker pull ${IMAGE_NAME}:${BRANCH_TAG} \
        || docker pull ${IMAGE_NAME}:main \
        || true

    DOCKER_BUILD_EXTRA_PARAMS=$(docker images \
        | grep ${IMAGE_NAME} \
        | head -n 1 \
        | awk '{ print "--cache-from=" $1 ":" $2 }' || true)
    docker build ${DOCKER_BUILD_EXTRA_PARAMS} -t ${IMAGE_NAME}:${CONTENT_HASH} domains/${DOMAIN_NAME}/services/${SERVICE_NAME}

    # Not stricly necessary as we are pushing the tags directly to the registry. However, it's still good
    # to set them as other scripts might be run after this one.
    docker tag ${IMAGE_NAME}:${CONTENT_HASH} ${IMAGE_NAME}:${COMMIT_TAG}
    docker tag ${IMAGE_NAME}:${CONTENT_HASH} ${IMAGE_NAME}:${BRANCH_TAG}

    docker push ${IMAGE_NAME}:${CONTENT_HASH}
fi

# Tag the images directly in the registry
docker buildx imagetools create ${IMAGE_NAME}:${CONTENT_HASH} \
    --tag ${IMAGE_NAME}:${COMMIT_TAG}
docker buildx imagetools create ${IMAGE_NAME}:${CONTENT_HASH} \
    --tag ${IMAGE_NAME}:${BRANCH_TAG}

I wrote a blog post about this script.