Switching the runtime base image with a run.Dockerfile

Platforms can have several run images available, each tailored to a specific language family - thus limiting the number of installed dependencies for each image to the minimum necessary to support the targeted language. Image extensions can be used to switch the run image to that most appropriate for the current application.

Examine curl extension


cat $PWD/samples/extensions/curl/bin/detect

The extension always detects (because its exit code is 0) and provides a dependency called curl.


cat $PWD/samples/extensions/curl/bin/generate

The extension generates a run.Dockerfile that switches the run image to reference localhost:5000/run-image-curl.

Build a run image for curl extension to use

cat $PWD/samples/base-images/alpine/run/curl.Dockerfile

This is a simple Dockerfile that creates a CNB run image from the curl base image by adding the required CNB user configuration and labels.

The Dockerfile could come from anywhere, but we include it in the base-images directory for convenience.

Build the run image:

docker build \
  --file $PWD/samples/base-images/alpine/run/curl.Dockerfile \
  --tag localhost:5000/run-image-curl .

docker push localhost:5000/run-image-curl

Configure the hello-extensions buildpack to require curl

Set the BP_REQUIRES build-time environment variable to configure the hello-extensions buildpack to require both vim and curl (review the ./bin/detect script to see why this works).

pack build hello-extensions \
  --builder localhost:5000/extensions-builder \
  --env BP_EXT_DEMO=1 \
  --env BP_REQUIRES=vim,curl \
  --path $PWD/samples/apps/java-maven \
  --pull-policy always \
  --network host \

Note that --network host is necessary when publishing to a local registry.

You should see:

[detector] ======== Results ========
[detector] pass: samples/vim@0.0.1
[detector] pass: samples/curl@0.0.1
[detector] pass: samples/cowsay@0.0.1
[detector] pass: samples/hello-extensions@0.0.1
[detector] Resolving plan... (try #1)
[detector] skip: samples/cowsay@0.0.1 provides unused cowsay
[detector] 3 of 4 buildpacks participating
[detector] samples/vim             0.0.1
[detector] samples/curl             0.0.1
[detector] samples/hello-extensions 0.0.1
[detector] Running generate for extension samples/vim@0.0.1
[detector] Running generate for extension samples/curl@0.0.1
[detector] Checking for new run image
[detector] Found a run.Dockerfile from extension 'samples/curl' setting run image to 'localhost:5000/run-image-curl'
[extender (build)] Found build Dockerfile for extension 'samples/vim'
[extender (build)] Applying the Dockerfile at /layers/generated/build/samples_vim/Dockerfile...
[extender (build)] Running build command
[extender (build)] ---> Hello Extensions Buildpack
[extender (build)] VIM - Vi IMproved 9.0 (2022 Jun 28, compiled May 19 2023 16:28:36)
Successfully built image hello-extensions

See the image run successfully

docker run --rm hello-extensions

You should see something akin to:

curl 7.85.0-DEV (x86_64-pc-linux-musl) ... more stuff here ...

What happened: now that hello-extensions requires both vim and curl in its build plan, both extensions are included in the build and provide the needed dependencies for build and launch, respectively

  • The vim extension installs vim at build time, as before
  • The curl extension switches the run image to localhost:5000/run-image-curl, which has curl installed

Now our curl process can succeed!

Next steps

Our curl process succeeded, but there is another process type defined on our image:

docker run --rm --entrypoint cowsay hello-extensions

You should see:

ERROR: failed to launch: path lookup: exec: "cowsay": executable file not found in $PATH

Our run image, localhost:5000/run-image-curl, has curl installed, but it doesn’t have cowsay.

In general, we may not always have a preconfigured run image available with all the needed dependencies for the current application. Luckily, we can also use image extensions to dynamically install runtime dependencies at build time. Let’s look at that next.

Next Step