Testing a Docker Image as part of a Jenkins Pipeline

Testing a Docker Image as part of a Jenkins Pipeline

  Published on January 15, 2021 ·   9 min read
  Author: Donald L. Schulz

One of the things that I have wanted to do is to test the structure of a docker image that I have just built. This should come right after the build stage so that if the structure is not what I expected, it would fail the build so that I don’t have a bunch of broken images sitting in my docker registry. I guess that this happens because I am using a docker image to build my project and the result of that output is copied into a clean docker image that will be my final image that can then be run as a docker container. The builder image should have been removed through this process but I have had things go wrong and then I don’t see the problem until I deploy it into my development environment. However, that then means that my broken image was published to my docker registry which is where the deployment would be pulling it from.

There are a couple of frameworks that you can use to make this happen and the one I am going to talk about today is the one from Google called container-structure-test. This code runs on a Linux or OS X platform which works out really well for me because I pretty much run all my Jenkins and build activities on Linux machines and containers. I will assume that you are building a Linux image and that my setup is similar to what you are currently doing. Later in this post I want to touch on running these container tests in Azure DevOps.

Where to Get the Framework

This is an open source framework hosted on GitHub: https://github.com/GoogleContainerTools/container-structure-test To install the binaries onto your Linux or OS X environment run the appropriate command:

curl -LO https://storage.googleapis.com/container-structure-test/latest/container-structure-test-darwin-amd64 && chmod +x container-structure-test-darwin-amd64 && sudo mv container-structure-test-darwin-amd64 /usr/local/bin/container-structure-test

curl -LO https://storage.googleapis.com/container-structure-test/latest/container-structure-test-linux-amd64 && chmod +x container-structure-test-linux-amd64 && mkdir -p $HOME/bin && export PATH=$PATH:$HOME/bin && mv container-structure-test-linux-amd64 $HOME/bin/container-structure-test

For those of you that have been following me for a while, I have this binary installed on my Jenkins container. I added the installation of the binary as an update to my docker jenkins-master image. Details of this are found in my Maintaining the Golden Images post.

The unit test

The test itself is just a yaml file with some value pairs set that are used to indicate to the testing framework what we are expecting and not expecting to see. Let me show you a test I have been using for my blog and then go over what this all means. There are four (4) types of tests that you can run with this tool.

  1. fileExistenceTests, which check to make sure a specific file (or directory) exist within the file system of the image. No contents of the files or directories are checked. These tests can also be used to ensure a file or directory is not present in the file system.
  2. fileContentTests, open a file on the file system and check its contents. These tests assume the specified file is a file, and that it exists (if unsure about either or these criteria, se the above fileExistenceTests). Regexes can again be used to check for expected or excluded content in the specified file.
  3. metadataTests, ensures the container is configured correctly.
  4. licenseTests, check a list of copyright files and makes sure all licenses are allowed at Google. By default it will look where Debian lists all copyright files, but can also look at an arbitrary list of files.
schemaVersion: '2.0.0'
  - name: 'Docker Entry Point Exists'
    path: '/docker-entrypoint.sh'
    shouldExist: true

  - name: 'index.html'
    path: '/usr/share/nginx/html/index.html'
    shouldExist: true

  - name: 'Source Files should not exist'
    path: '/app/blog/'
    shouldExist: false

  - name: 'Hexo Link should not exist'
    path: '/usr/local/bin/hexo'
    shouldExist: false

  - name: 'Hexo should not exist'
    path: '/usr/local/lib/node_modules/hexo-cli/bin/Hexo'
    shouldExist: false

As you can see from this test file that I am just using the fileExistenceTests for all my tests as this was good enough to get me started on testing my images and represented the most issues that I would run into when things did not go well. For more information on the various test types that I am not covering here checkout source and documentation on github.

Now that we have all the pieces that we need lets get back to our Jenkins pipeline and setup a step to run this test and either move on to the next stage or fail the build if there is a problem with the image. I am going to make another assumption that you are familiar with Jenkins declarative pipelines. If this is all new to you, check out my previous post Pipeline As Code

Testing container-structure-test

If you run the code against a fresh image the results of the docker build command the syntax is pretty simple to get a report on the image.

container-structure-test test --image '<replace with image name>' --config './test/DockerTest/unit-test.yaml'

Docker Test Results
As you can see from just running the program locally from my WSL Ubuntu distro on my Windows 10 machine I get a really nice report which would tell me which tests passed and which failed. In this case I was successful and all of my tests have passed.

The real challenge in getting this to run within the context of a Jenkins pipeline is somehow letting Jenkins know that the tests failed when they do. If we just ran this in a shell script it would just carry on because there is no exceptions so all must be good. We need to introduce a couple of additional elements to this story. Instead of getting a nice report like this we need to return the results as json and then pipe that into something that can parse json and return the number of failures. Anything that is larger than 0 is a failure.

Introducing Jq

Jq is a json parser and if you are using a distro that uses apt-get to install software like Ubuntu for your Jenkins builder then you can probably install this open source software with the following command:

sudo apt-get update -y
sudo apt-get install -y jq

If you follow this link https://stedolan.github.io/jq/download/ where there are a number of platforms of how you can install jq to the Jenkins build machine. Now, if that won’t work for you than you can download the binaries and install manually from their github repository found at https://github.com/stedolan/jq/releases

Test-Image Stage

Now we are ready to put this all together we have all the pieces that we need. For this step in our Jenkins declarative Pipeline we are going to write this as a groovy script. This part took me the longest to figure out as I could not get the kind of reporting that I needed from a shell script and finally came up with the following work flow. Here is the step in its entirety.

    steps {
        script {
            try {                           
                def status = 0
                status = sh(returnStdout: true, script: "container-structure-test test --image '<name of your image>' --config './test/DockerTest/unit-test.yaml' --json | jq .Fail") as Integer
                if (status != 0) {                            
                    error 'Image Test has failed'

            } catch (err) {
                error "Test-Image ERROR: The execution of the container structure tests failed, see the log for details."
                echo err

At line 4 we have the meat of the script wrapped in a try catch so that we can also capture any exceptions that might occur. Line 5 we define a variable named status and give it a value of 0. We start our test assuming that everything is good, as you remember from earlier that we are returning the number of errors so if we get 0 there were none.

Line 6 is very busy as I am using an inline shell command to execute the actual command. First off I am setting it to return a standard out and then the second half is the script that we are calling. This command is just like the one we were calling when we were running this command manually expect that we have added once more option –json which will return the results as a json file. Then we pipe the results of the json file to the jq program and let it know we want to parse the value for Fail. If there are no errors then the Fail value will be zero. By default the value is returned as a string so we do a conversion on the fly so that it will be an integer.

Line 7 we check the value of status which is were the integer value from jq was assigned to. If this is not zero (0) we set an error and this is enough from Jenkins to fail this build and the pipeline will stop here as a failure.

There are a lot of moving pieces and does get a little bit complicated to put this all together, but it is so worth it not having bad docker images in my docker repositories. This is also a good example of how much harder and more complicated it is to get this working with this kind of setup. It is a whole lot easier to set this up on Azure DevOps because this all works right out of the box, don’t even need any additional extensions.

What about Azure DevOps

I am glad you asked as there is a task that does just that. In your Azure pipeline if you search for “Container Structure Test” you should see this task as it is built into the regular tasks that are part of the build. You don’t have to go looking for it in the Marketplace it should already be available to you.

However, it turns out that is not quite the case here. I did try to work with this task but could not get it to work it would complain about missing libraries and I attempted to install those before running the command but it just would not co-operate. However, this tool also has some serious limitations compared to the solution I have running in a Jenkins pipeline. My goal was to always test these images before I publish them to a repository but this tool would only work with a repository and sort of defeats the purpose. Even the docker control has a build and publish step to it but guess where your finished work is, in the repository before you even know if it is good or not.

Some months ago I started moving all my projects and infrastructure away from Azure DevOps and into GitHub and using Jenkins as my Pipeline tool. Initially I took this approach to get more experience with this but I also added building everything into docker images and running them as Docker containers as cheaply as possible and that was not going to be a cloud managed service like AKS as this was just too much money for what I wanted to do. I find I am having more flexibility in how I am building my docker containers as my Jenkins Server is running in a Docker Container and I am building a Docker Image and run that image on the same container for my test environment. I can’t see how I could even come close to doing something like that in Azure unless I go full Azure Kubernetes Service (AKS) and that is just too expensive.