default only cache one time

.travis.yml

@@ -1,22 +1,24 @@
 language: bash
+
 sudo: required
+services:
+  - docker
+
+script:
+  - docker build --rm --cache-from boot2docker/boot2docker  -t boot2docker/boot2docker .
+  - docker run --rm boot2docker/boot2docker > ${TRAVIS_BUILD_DIR}/boot2docker.iso
+  - if [ -z "${DEPLOY_ISO_TO_GITHUB}" ] ; then  export DEPLOY_ISO_TO_GITHUB=false ; fi
 
 cache:
   bundler: true
   directories:
   - $HOME/docker
 before_cache:
-- if [  ! "${NOT_CACHE_IMG_AFTER_BUILD}" = "true"  ] ; then   mkdir -p $HOME/docker && docker images   --filter='dangling=false' --format '{{.Repository}}:{{.Tag}}' | grep -v none | xargs -n 1 -I {x} -t sh -c 'docker save {x} | gzip -2 > $HOME/docker/$(echo {x} | md5sum | awk "{print \$1}" ).tar.gz' ; echo ok ;fi
+- imglist=$(find  $HOME/docker/ -name "*.tar.gz");test -z "$imglist"  &&  test ! "$FORCE_CACHE_IF_NOT" = "false"  && NOT_CACHE_IMG_AFTER_BUILD=false ; test -n "$imglist"  &&  test -z "$NOT_CACHE_IMG_AFTER_BUILD" && NOT_CACHE_IMG_AFTER_BUILD=true ; echo "$imglist"; if [  ! "${NOT_CACHE_IMG_AFTER_BUILD}" = "true"  ] ;  then   mkdir -p $HOME/docker && docker images   --filter='dangling=false' --format '{{.Repository}}:{{.Tag}}' | grep -v none | xargs -n 1 -I {x} -t sh -c 'docker save {x} | gzip -2 > $HOME/docker/$(echo {x} | md5sum | awk "{print \$1}" ).tar.gz' ; echo ok ;fi
 - bash -c " ls -lA $HOME/docker/ "
 before_install:
 - if [ ! "${NOT_LOAD_CACHE_BEFORE_BUILD}" = "true"  ] ; then  if [[ -d $HOME/docker ]]; then  imglist=$(find  $HOME/docker/ -name "*.tar.gz"); echo "$imglist"; echo "${imglist}" | xargs -t -I {file} sh -c "zcat {file} | docker load";  fi ;  bash -c "docker images -a --filter='dangling=false'" ; fi
 
-services:
-- docker
-script:
-- docker build --rm --cache-from boot2docker/boot2docker  -t boot2docker/boot2docker .
-- docker run --rm boot2docker/boot2docker > ${TRAVIS_BUILD_DIR}/boot2docker.iso
-- if [ -z "${DEPLOY_ISO_TO_GITHUB}" ] ; then  export DEPLOY_ISO_TO_GITHUB=false ; fi
 after_script:
   - docker images
   - ls -l *.iso