Redesign integration testing to copy images to nested containers

Previously, the strategy for avoiding lots of rebuilding and repulling
for each Docker version being tested was to use a mountpoint to persist
/var/lib/docker. This was pretty broken, and may not be a reliable
strategy. This commit changes the scripts to instead build/pull images
outside the innermost container, and copy them to the final test
environment with docker save/docker load.

This requires a fair amount of changes, since run.sh must now
communicate with the Docker engine that was formerly started by
test_runner.sh. The code that starts this engine has been broken out to
run_engine.sh so that starting the engine and running the tests under it
can be done separately (with the images loaded in between these steps).

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

contrib/docker-integration/helpers.bash

@@ -0,0 +1,21 @@
+# Start docker daemon
+function start_daemon() {
+	# Drivers to use for Docker engines the tests are going to create.
+	STORAGE_DRIVER=${STORAGE_DRIVER:-overlay}
+	EXEC_DRIVER=${EXEC_DRIVER:-native}
+
+	docker --daemon --log-level=panic \
+		--storage-driver="$STORAGE_DRIVER" --exec-driver="$EXEC_DRIVER" &
+	DOCKER_PID=$!
+
+	# Wait for it to become reachable.
+	tries=10
+	until docker version &> /dev/null; do
+		(( tries-- ))
+		if [ $tries -le 0 ]; then
+			echo >&2 "error: daemon failed to start"
+			exit 1
+		fi
+		sleep 1
+	done
+}