Install Docker

Here is a lean version of the Docker site content that I tested on Centos 7.5.  It yum installs some pre-requisites, adds the stable Docker Community Edition repository to yum, and then installs and starts Docker.

sudo yum install -y yum-utils \
device-mapper-persistent-data \
lvm2
sudo yum-config-manager \
--add-repo \
https://download.docker.com/linux/centos/docker-ce.repo
sudo yum install docker-ce
sudo systemctl start docker

Now Docker is started – but only the root user can really use it.  So, let’s create the docker group and add our current user to it.  That way we can use docker with our current user and avoid having to use sudo on every command.

These instructions from from here: https://docs.docker.com/install/linux/linux-postinstall/#manage-docker-as-a-non-root-user.

sudo groupadd docker
sudo usermod -aG docker $USER

After this, please re-log in (e.g. exit out of SSH and jump back into your server) so that your group memberships apply.

Now Docker is running and we can use it as ourselves.

Get Terraform Working in Docker

We will run Terraform as a single command inside of a Docker image.  So, let’s start by getting the latest Terraform image form Hashicorp:

docker pull hashicorp/terraform

Create a directory for your Terraform work and give ownership to your user. Also create a sub-directory to act as the Docker volume in which we will put your Terraform plans.

sudo mkdir /opt/terraform && sudo chown $USER:$USER /opt/terraform
cd /opt/terraform
mkdir tf-vol

Now let’s create a file at /opt/terraform/tf-vol/plan.tf with a sample Terraform plan (just a debug one).

output "test" {
  value = "Hello World!"
}

After this, we can run Terraform and tell docker to use that tf-vol directory as as a volume. Terraform will use it as the working directory, will find our plan, and will display “Hello World!”.

$ docker run -i -t -v /opt/terraform/tf-vol:/tf-vol/ -w /tf-vol/ hashicorp/terraform:light apply

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Outputs:

test = Hello World!

So, we now have Docker installed, and Terraform running with it using an external volume to store our plans.