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Generating a build.Dockerfile

Examine tree extension

detect

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

The extension always detects (because its exit code is 0) and provides a dependency called tree by writing to the build plan.

generate

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

The extension generates a build.Dockerfile that installs tree on the builder image.

Re-create our builder with hello-extensions updated to require tree

Edit $PWD/samples/buildpacks/hello-extensions/bin/detect to uncomment the first set of lines that output [[requires]] to the build plan:

sed -i "10,11s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect

(On Mac, use sed -i '' "10,11s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect)

Re-create the builder:

pack builder create localhost:5000/extensions-builder \
  --config $PWD/samples/builders/alpine/builder.toml \
  --publish

Re-build the application image

pack build hello-extensions \
  --builder localhost:5000/extensions-builder \
  --network host \
  --path $PWD/samples/apps/java-maven \
  --pull-policy always \
  --verbose

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

You should see:

[detector] ======== Results ========
[detector] pass: samples/tree@0.0.1
[detector] pass: samples/hello-extensions@0.0.1
[detector] Resolving plan... (try #1)
[detector] samples/tree             0.0.1
[detector] samples/hello-extensions 0.0.1
[detector] Running generate for extension samples/tree@0.0.1
...
[extender] Found build Dockerfile for extension 'samples/tree'
[extender] Applying the Dockerfile at /layers/generated/build/samples_tree/Dockerfile...
...
[extender] Running build command
[extender] ---> Hello Extensions Buildpack
[extender] tree v1.8.0 (c) 1996 - 2018 by Steve Baker, Thomas Moore, Francesc Rocher, Florian Sesser, Kyosuke Tokoro
...
Successfully built image hello-extensions

See the image fail to run

docker run --rm hello-extensions

You should see:

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

What happened: our builder uses run image cnbs/sample-stack-run:alpine, which does not have curl installed, so our process failed to launch.

Let’s take a look at how the samples/curl extension fixes the error by switching the run image to another image…

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