Make builpacks task Chains compliant

[rgreinho]

Adds the Chains type hinting to the task to ensure it can be used by Chains.

This problem came up based on the following issue in the Chains repository https://github.com/tektoncd/chains/issues/273#issuecomment-956411302.

[jromero]

Diff between 0.3 and 0.4

diff --git a/./task/buildpacks/0.3/README.md b/./task/buildpacks/0.4/README.md
index 2b1617d..ac1b11e 100644
--- a/./task/buildpacks/0.3/README.md
+++ b/./task/buildpacks/0.4/README.md
@@ -43,6 +43,7 @@ kubectl apply -f https://raw.githubusercontent.com/buildpacks/tekton-integration
 ## Results

  - **`APP_IMAGE_DIGEST`**: The digest of the built `APP_IMAGE`.
+ - **`APP_IMAGE_URL`**: The URL of the built `APP_IMAGE`.

 ## Builders

@@ -100,4 +101,4 @@ For support of previous [Platform API][platform-api]s use a previous version of
 | [0.2](../0.2/) | [0.3][platform-api-0.3]

 [platform-api]: https://buildpacks.io/docs/reference/spec/platform-api/
-[platform-api-0.3]: https://github.com/buildpacks/spec/blob/platform/0.3/platform.md
\ No newline at end of file
+[platform-api-0.3]: https://github.com/buildpacks/spec/blob/platform/0.3/platform.md
diff --git a/./task/buildpacks/0.3/README.tpl.md b/./task/buildpacks/0.4/README.tpl.md
index f20a8da..44283de 100644
--- a/./task/buildpacks/0.3/README.tpl.md
+++ b/./task/buildpacks/0.4/README.tpl.md
@@ -59,4 +59,4 @@ For support of previous [Platform API][platform-api]s use a previous version of
 | [0.2](../0.2/) | [0.3][platform-api-0.3]

 [platform-api]: https://buildpacks.io/docs/reference/spec/platform-api/
-[platform-api-0.3]: https://github.com/buildpacks/spec/blob/platform/0.3/platform.md
\ No newline at end of file
+[platform-api-0.3]: https://github.com/buildpacks/spec/blob/platform/0.3/platform.md
diff --git a/./task/buildpacks/0.3/buildpacks.yaml b/./task/buildpacks/0.4/buildpacks.yaml
index 4ccc867..b919360 100644
--- a/./task/buildpacks/0.3/buildpacks.yaml
+++ b/./task/buildpacks/0.4/buildpacks.yaml
@@ -4,7 +4,7 @@ kind: Task
 metadata:
   name: buildpacks
   labels:
-    app.kubernetes.io/version: "0.3"
+    app.kubernetes.io/version: "0.4"
   annotations:
     tekton.dev/categories: Image Build
     tekton.dev/pipelines.minVersion: "0.17.0"
@@ -60,6 +60,8 @@ spec:
   results:
     - name: APP_IMAGE_DIGEST
       description: The digest of the built `APP_IMAGE`.
+    - name: APP_IMAGE_URL
+      description: The URL of the built `APP_IMAGE`.

   stepTemplate:
     env:
@@ -105,7 +107,7 @@ spec:
         mkdir -p "$ENV_DIR"

         for env in "${envs[@]}"; do
-            IFS='=' read -r key value string <<< "$env"
+            IFS='=' read -r key value <<< "$env"
             if [[ "$key" != "" && "$value" != "" ]]; then
                 path="${ENV_DIR}/${key}"
                 echo "--> Writing ${path}..."
@@ -152,7 +154,11 @@ spec:
       script: |
         #!/usr/bin/env bash
         set -e
-        cat /layers/report.toml | grep "digest" | cut -d'"' -f2 | cut -d'"' -f2 | tr -d '\n' | tee $(results.APP_IMAGE_DIGEST.path)
+        grep "digest" /layers/report.toml | cut -d'"' -f2 | cut -d'"' -f2 | tr -d '\n' | tee "$(results.APP_IMAGE_DIGEST.path)"
+
+        # Disable shellcheck here since $() is use for variable substitution and not command execution.
+        # shellcheck disable=SC2005
+        echo "$(params.APP_IMAGE)" | tee "$(results.APP_IMAGE_URL.path)"
       volumeMounts:
         - name: layers-dir
           mountPath: /layers

[jromero]

Thank you @rgreinho for the great work here. Very much appreciated.

[jjbustamante]

Great work @rgreinho! thanks for the contribution.