travis docker layers cache

.travis.yml

@@ -12,6 +12,11 @@ env:
     - MINOR=$(cut -d. -f2 .semver)
     - PATCH=$(cut -d. -f3 .semver)
 
+cache:
+  bundler: true
+  directories:
+    - $HOME/docker
+
 jobs:
   include:
     - stage: build x64 image
@@ -78,6 +83,15 @@ jobs:
       before_script:
         - ARCH=armhf
 
+      before_cache:
+        # Save tagged docker images
+        - >
+          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'
+      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
+      
       script:
         # prepare qemu
         - docker run --rm --privileged multiarch/qemu-user-static:register --reset