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Just-in-Time Containers

Just-in-time containers are produced by calling a Dagger Function that returns the Container type. This type provides a complete API for building, running and distributing containers.

Just-in-time containers might be produced by a Dagger Function that:

  • Builds a container
  • Minifies a container
  • Downloads a container image from a running registry
  • Exports a container from Docker or other container runtimes
  • Snapshots the state of a running container

You can think of a just-in-time container, and the Container type that represents it, as a build stage in Dockerfile. Each operation produces a new immutable state, which can be further processed, or exported as an OCI image. Dagger Functions can accept, return and pass containers between themselves, just like regular variables.

Here is an example of a Wolfi container builder Dagger Function that returns a base Wolfi container image with a list of additional specified packages:

dagger -m github.com/shykes/daggerverse/wolfi@v0.1.4 call container --packages=cowsay

Once the command completes, you should see this output:

_type: Container
defaultArgs:
    - /bin/sh
entrypoint: []
mounts:
    - /tmp
    - /dev/shm
platform: linux/arm64
user: ""
workdir: /bass/work

This means that the build succeeded, and a Container type representing the built container image was returned.