message: chore: cache Travis CI docker builds
  Let's see if we can reduce the time it gets to build and test Etcher on
  GNU/Linux.
  See:
diff: @@ -16,6 +16,7 @@ cache:
  - $HOME/Library/Caches/electron
  - $HOME/Library/Caches/electron-builder
  - $HOME/.pkg-cache
  + - $HOME/docker
  - node_modules
  services:
  @@ -35,6 +36,7 @@ env:
  - CC="clang"
  - CXX="clang++"
  - HOMEBREW_NO_AUTO_UPDATE=1
  + - DOCKER_CACHE_FILE=$HOME/docker/cache.tar.gz
  matrix:
  - TARGET_ARCH=x64
  - TARGET_ARCH=x86
  @@ -50,6 +52,9 @@ os:
  - osx
  before_install:
  + - if [ -f ${DOCKER_CACHE_FILE} ] && [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
  + gunzip -c ${DOCKER_CACHE_FILE} | docker load;
  + fi
  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then PATH=/usr/local/opt/ccache/libexec:$PATH; fi
  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
  export HOST_OS="darwin";
  @@ -63,6 +68,10 @@ install:
  script:
  - ./scripts/ci/test.sh -o $HOST_OS -r $TARGET_ARCH
  - ./scripts/ci/build-installers.sh -o $HOST_OS -r $TARGET_ARCH
  + - if [[ ${TRAVIS_BRANCH} == "master" ]] && [[ ${TRAVIS_PULL_REQUEST} == "false" ]]; then
  + mkdir -p $(dirname ${DOCKER_CACHE_FILE});
  + docker save $(docker history -q ${DOCKER_REPOSITORY}:${TRAVIS_COMMIT} | grep -v '<missing>') | gzip > ${DOCKER_CACHE_FILE};
  + fi
  deploy:
  provider: script
  