upgrade docker and reenable buildkit

.travis.yml

@@ -1,6 +1,11 @@
 language: minimal
 services:
 - docker
+before_install:
+- curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
+- sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
+- sudo apt-get update
+- sudo apt-get -y -o Dpkg::Options::="--force-confnew" install docker-ce
 addons:
   apt:
     update: true
@@ -13,7 +18,6 @@ addons:
 env:
   global:
     - PARALLELMFLAGS="-j2"
-    - DOCKER_BUILDKIT=0
   matrix:
     - DISTRIB_ID=ubuntu VARIANT=debug        MARCH=x86_64   CHECK_TARGET=memcheck
     - DISTRIB_ID=ubuntu VARIANT=coverage     MARCH=x86_64   CHECK_TARGET=check