cleanup docker cache before run, see debuerreotype/docker-debian-artifacts#69

Author: k-okada

.travis.yml

@@ -54,6 +54,8 @@ before_install: # Use this to prepare the system to install prerequisites or dep
 install:
   - export CI_SOURCE_PATH=$(pwd)
   - export REPOSITORY_NAME=${PWD##*/}
+  - yes | docker image prune
+  - yes | docker container prune
   - if [[ "$QEMU" != "" ]]; then sudo apt-get install -y -qq qemu-user-static; ls /usr/bin/qemu-*-static; export QEMU_VOLUME="-v /usr/bin/qemu-$QEMU-static:/usr/bin/qemu-$QEMU-static" ; fi
   - if [[ "$QEMU" != "" ]]; then docker run --rm --privileged multiarch/qemu-user-static:register; fi
   - if [[ "$DOCKER_IMAGE" == *"arm"* ]]; then sudo apt-get install -y -qq qemu-user-static; fi