This is a prove of concept for leveraging the travis directory caching for speeding up docker builds. It works by configuring the docker deamon to use a folder under current user's (travis) control. That way you have the privileges to use the caching feature of travis ci.

.travis.yml

sudo: false

services:
  - docker

before_script:
  - sudo service docker stop
  - if [ "$(ls -A /home/travis/docker)" ]; then echo "/home/travis/docker already set"; else sudo mv /var/lib/docker /home/travis/docker; fi
  - sudo bash -c "echo 'DOCKER_OPTS=\"-H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock -g /home/travis/docker\"' > /etc/default/docker"
  - sudo service docker start
  - docker build -f Dockerfile-testenv -t testenv .

script:
  - docker run testenv

before_cache:
  - sudo service docker stop
  - sudo chown -R travis ~/docker

cache:
  directories:
    ~/docker

@rmcdaniel it's possible to specify a longer cache timeout in .travis.yml if this indeed speeds up your process. At some point the amount of time it takes to tar up the files and push / pull ends up being more than just pulling down fresh each time though. (We discovered on caches > 1GB).

+1

• Instead, cache the output of docker save and load from cache using docker load.
• Or docker push/pull to/from hub.docker.com (after securely setting $DOCKER_PASSWORD in travis)
• but only do this if (re)building layers is really slower than downloading

services:
  - docker

before_script:
  - mkdir -p ~/docker-images
  - [[ -f ~/docker-images/testenv.tgz ]] && docker load -i ~/docker-images/testenv.tgz
  # or: - docker pull casperdcl/testenv
  - docker build -f Dockerfile-testenv -t casperdcl/testenv .
  - docker save casperdcl/testenv | gzip -c > ~/docker-images/testenv.tgz
  # or: - echo "$DOCKER_PASSWORD" | docker login -u casperdcl --password-stdin
  # - docker push casperdcl/testenv

script:
  - docker run casperdcl/testenv

cache:
  directories:
    ~/docker-images