By default, jobs run on the host machine where the agent is installed. This is convenient and typically well-suited for projects that are just beginning to adopt continuous integration (CI). Over time, you may find that you want more control over the stage where your tasks run.
On Linux and Windows agents, jobs may be run on the host or in a container. (On macOS and Red Hat Enterprise Linux 6, container jobs are not available.) Containers provide isolation from the host and allow you to pin specific versions of tools and dependencies. Host jobs require less initial setup and infrastructure to maintain.
Containers offer a lightweight abstraction over the host operating system. You can select the exact versions of operating systems, tools, and dependencies that your build requires. When you specify a container in your pipeline, the agent will first fetch and start the container. Then, each step of the job will run inside the container.
The Azure Pipelines system requires a few things in Linux-based containers:
- Bash (for the
bashstep / task, which most container pipelines will use)
- Can run Node.js (which the agent provides)
- Does not define an
And on your agent host:
- Ensure Docker is installed
- The agent must have permission to access the Docker daemon
Be sure your container has each of these tools available. Some of the extremely stripped-down
containers available on Docker Hub, especially those based on Alpine Linux, don't satisfy these
minimum requirements. Containers with a
ENTRYPOINT might not work, since Azure Pipelines
docker create an awaiting container and
docker exec a series of commands which expect
the container is always up and running.
Also note: the Red Hat Enterprise Linux 6 build of the agent won't run container job. Choose another Linux flavor, such as Red Hat Enterprise Linux 7 or above.
ubuntu-16.04 pools support running containers.
The Hosted macOS pool does not support running containers.
A simple example:
pool: vmImage: 'ubuntu-16.04' container: ubuntu:16.04 steps: - script: printenv
This tells the system to fetch the
ubuntu image tagged
Docker Hub and then start the container. When the
printenv command runs, it will happen inside the
You must specify "Hosted Ubuntu 1604" as the pool name in order to run Linux containers. Other pools won't work.
A Windows example:
pool: vmImage: 'win1803' container: mcr.microsoft.com/windows/servercore:1803 steps: - script: set
Windows requires that the kernel version of the host and container match.
Since this example uses the hosted Windows Container pool, which is running an 1803
build, we also use the
1803 tag for the container.
Containers are also useful for running the same steps in multiple jobs.
In the following example, the same steps run in multiple versions of Ubuntu Linux.
(And we don't have to mention the
jobs keyword, since there's only a single job defined.)
pool: vmImage: 'ubuntu-16.04' strategy: matrix: ubuntu14: containerImage: ubuntu:14.04 ubuntu16: containerImage: ubuntu:16.04 ubuntu18: containerImage: ubuntu:18.04 container: $[ variables['containerImage'] ] steps: - script: printenv
Containers can be hosted on registries other than Docker Hub. To host an image on Azure Container Registry or another private container registry, add a service connection to the private registry. Then you can reference it in a container spec:
container: image: myprivate/registry:ubuntu1604 endpoint: private_dockerhub_connection steps: - script: echo hello
container: image: myprivate.azurecr.io/windowsservercore:1803 endpoint: my_acr_connection steps: - script: echo hello
If you need to control container startup, you can specify
container: image: ubuntu:16.04 options: --hostname container-test --ip 192.168.0.1 steps: - script: echo hello
Reusable container definition
In the following example, the containers are defined in the resources section.
Each container is then referenced later, by referring to its assigned alias.
(Here, we explicitly list the
jobs keyword for clarity.)
resources: containers: - container: u14 image: ubuntu:14.04 - container: u16 image: ubuntu:16.04 - container: u18 image: ubuntu:18.04 jobs: - job: RunInContainer pool: vmImage: 'ubuntu-16.04' strategy: matrix: ubuntu14: containerResource: u14 ubuntu16: containerResource: u16 ubuntu18: containerResource: u18 container: $[ variables['containerResource'] ] steps: - script: printenv
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