Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

[zbxzxx]

my OS is ubuntu 16.04

when i invoke the jenkins build ,i find the result

Started by user admin
Building in workspace /tmp/jenkins-buildenv/Docker_test-job/workspace
No credentials specified

git rev-parse --is-inside-work-tree # timeout=10
Fetching changes from the remote Git repository
git config remote.origin.url https://github.com/jamtur01/docker-jenkins-sample.git # timeout=10
Fetching upstream changes from https://github.com/jamtur01/docker-jenkins-sample.git
git --version # timeout=10
git fetch --tags --progress https://github.com/jamtur01/docker-jenkins-sample.git +refs/heads/:refs/remotes/origin/
git rev-parse refs/remotes/origin/master^{commit} # timeout=10
git rev-parse refs/remotes/origin/origin/master^{commit} # timeout=10
Checking out Revision 7d027a4 (refs/remotes/origin/master)
git config core.sparsecheckout # timeout=10
git checkout -f 7d027a4
Commit message: "Update Dockerfile"
git rev-list --no-walk 7d027a4 # timeout=10
[workspace] $ /bin/sh -xe /tmp/jenkins651930525195439723.sh

• sudo docker build .
• tail -1
• awk { print $NF }
  Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
• IMAGE=
• MNT=/tmp/jenkins-buildenv/Docker_test-job/workspace/..
• sudo docker run -d -v /tmp/jenkins-buildenv/Docker_test-job/workspace/..:/opt/project/ /bin/bash -c cd /opt/project/workspace && rake spec
  docker: invalid reference format.
  See 'docker run --help'.
• CONTAINER=
  Build step 'Execute shell' marked build as failure
  Recording test results
  ERROR: Step ‘Publish JUnit test result report’ failed: No test report files were found. Configuration error?
  Finished: FAILURE

then i see
https://docs.docker.com/install/linux/docker-ce/binaries/

Before attempting to install Docker from binaries, be sure your host machine meets the prerequisites:

A 64-bit installation
Version 3.10 or higher of the Linux kernel. The latest version of the kernel available for your platform is recommended.
iptables version 1.4 or higher
git version 1.7 or higher
A ps executable, usually provided by procps or a similar package.
XZ Utils 4.9 or higher
A properly mounted cgroupfs hierarchy; a single, all-encompassing cgroup mount point is not sufficient. See Github issues #2683, #3485, #4568).

then i use
docker exec -t -t jenkins /bin/bash
and
sudo apt-get install iptables
then it works