added DOCKER_VERSION variable

.semaphoreci/setup.sh

@@ -1,6 +1,10 @@
 #!/usr/bin/env bash
 set -e
 
+sudo -E apt-get -yq update
+sudo -E apt-get -yq --no-install-suggests --no-install-recommends --force-yes install docker-engine=${DOCKER_VERSION}*
+docker version
+
 pip install --user -r requirements.txt
 
 make pull-images

.semaphoreci/vars

@@ -5,6 +5,8 @@ export secure='btt4r13t09gQlHb6gYrvGC2yGCMMHfnp1Mz1RQedc4Mpf/FfT8aE6xmK2a2i9CCvs
 
 export REPO='containous/traefik'
 
+export DOCKER_VERSION=1.12.6
+
 if VERSION=$(git describe --exact-match --abbrev=0 --tags);
 then
   export VERSION