Generating a run.Dockerfile that extends the runtime base image

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 |
                ||     ||