Extending the runtime base image with a run.Dockerfile

Run images can be kept lean if image extensions are used to dynamically install the needed dependencies for the current application.

Examine cowsay extension


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

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


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

The extension generates a run.Dockerfile that installs cowsay on the current run image.

Configure the hello-extensions buildpack to require cowsay

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,cowsay \
  --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] samples/vim             0.0.1
[detector] samples/curl             0.0.1
[detector] samples/cowsay           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] Running generate for extension samples/cowsay@0.0.1
[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 Dockerfile at /layers/generated/build/samples_vim/Dockerfile...
[extender (run)] Found run Dockerfile for extension 'samples/curl'
[extender (run)] Found run Dockerfile for extension 'samples/cowsay'
[extender (run)] Applying Dockerfile at /layers/generated/run/samples_curl/Dockerfile...
[extender (run)] Applying Dockerfile at /layers/generated/run/samples_cowsay/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

Note: build image extension and run image extension are done in parallel, so the log lines for those phases may print in a different order from that shown above.

See the image run successfully

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

You should see something akin to:

< MOOOO! >
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||