Disable TLS for e2e docker-in-docker daemon

The docker-in-docker image now enables TLS by default (added in
docker-library/docker#166), which complicates testing in our
environment, and isn't needed for the tests we're running.

This patch sets the `DOCKER_TLS_CERTDIR` to an empty value to
disable TLS.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit b1a3c1aad1)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

e2e/compose-env.yaml

@@ -8,6 +8,8 @@ services:
       image: 'docker:${TEST_ENGINE_VERSION:-stable-dind}'
       privileged: true
       command: ['--insecure-registry=registry:5000']
+      environment:
+        - DOCKER_TLS_CERTDIR=
 
   notary-server:
       build: