Skaffold’s Docker network build feature lets you isolate your build environment, but it’s often misunderstood as just a way to "speed things up."
Let’s see Skaffold in action, building an image with a custom Docker network. Imagine this skaffold.yaml:
apiVersion: skaffold/v2beta29
kind: Config
build:
artifacts:
- image: my-app
docker:
network: my-build-network
local: {}
deploy:
kubectl:
manifests:
- k8s/*
And a Dockerfile like this:
FROM ubuntu:22.04
RUN apt-get update && apt-get install -y --no-install-recommends curl
RUN curl -sSL https://example.com/some-dependency.tar.gz | tar xz
COPY . /app
WORKDIR /app
CMD ["./my-app"]
When you run skaffold dev, Skaffold will first ensure a Docker network named my-build-network exists. If it doesn’t, Docker creates it. Then, Skaffold starts a temporary container, attaches it to my-build-network, and executes your docker build command inside that container. The build context is mounted into this temporary container. Finally, the built image is tagged and pushed (or saved locally).
The core problem Skaffold’s custom Docker network for builds solves is managing dependencies and network isolation during the image build process itself. Without it, your docker build command runs directly on your host’s Docker daemon, which might have network access issues, or it might inadvertently pull dependencies from your host’s local network that you don’t want in your image’s reproducible build. This feature allows you to define a clean, isolated network for the build, ensuring that the only network access the build container has is what you explicitly allow or what’s available on the custom network. This is crucial for reproducible builds and for preventing "it works on my machine" scenarios where hidden network access is the culprit.
The exact levers you control are primarily within the skaffold.yaml docker.network field. You specify the name of the Docker network. This network can be pre-existing on your Docker daemon, or Skaffold can create it if it doesn’t exist. If you need to connect to services running within that custom network during the build (e.g., a private artifact repository), you’d start those services in containers attached to the same network before running skaffold dev. Skaffold’s build container will then be able to resolve and connect to them by their container names on that network.
What many folks miss is that the Docker build itself happens inside a temporary container managed by Skaffold, not directly on your host’s Docker daemon. This temporary container is then attached to the specified network. This means any RUN commands in your Dockerfile that perform network operations (like curl or apt-get) will be using the network interface of this temporary build container, not your host’s direct network. This isolation is the key to predictable builds.
The next concept you’ll likely explore is how to manage private registries and authentication for dependencies pulled during the build process when using isolated networks.