Add docker image caching

.travis.yml

@@ -1,10 +1,28 @@
 sudo: required
+
+before_cache:
+  # Save tagged docker images see https://github.com/travis-ci/travis-ci/issues/5358#issuecomment-248915326
+  - >
+    mkdir -p $HOME/docker && docker images -a --filter='dangling=false' --format '{{.Repository}}:{{.Tag}} {{.ID}}'
+    | xargs -n 2 -t sh -c 'test -e $HOME/docker/$1.tar.gz || docker save $0 | gzip -2 > $HOME/docker/$1.tar.gz'
+
+cache:
+  bundler: true
+  directories:
+    - $HOME/docker
+
 services:
 - docker
+
+before_install:
+  # Load cached docker images
+  - if [[ -d $HOME/docker ]]; then ls $HOME/docker/*.tar.gz | xargs -I {file} sh -c "zcat {file} | docker load"; fi
+
 addons:
   apt:
     packages:
     - p7zip-full
+
 before_script:
 - docker build . -t wups-loader-builder
 script: