Improve extensions demo by not requiring the builder to be recreated …

…after every failed build

The hello-extensions buildpack can be configured through the environment after buildpacks/samples#142 lands

Signed-off-by: Natalie Arellano <narellano@vmware.com>

content/docs/extension-author-guide/create-extension/build-dockerfile.md

@@ -25,32 +25,16 @@ 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`
+### Configure the `hello-extensions` buildpack 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:
-
-<!-- test:exec -->
-```bash
-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:
-
-<!-- test:exec -->
-```
-pack builder create localhost:5000/extensions-builder \
-  --config $PWD/samples/builders/alpine/builder.toml \
-  --publish
-```
-
-### Re-build the application image
+Set the `BP_REQUIRES` build-time environment variable to configure the `hello-extensions` buildpack to require `tree` (review the `./bin/detect` script to see why this works).
 
 <!-- test:exec -->
 ```
 pack build hello-extensions \
   --builder localhost:5000/extensions-builder \
+  --env BP_EXT_DEMO=1 \
+  --env BP_REQUIRES=tree \
   --network host \
   --path $PWD/samples/apps/java-maven \
   --pull-policy always \

content/docs/extension-author-guide/create-extension/run-dockerfile.md

@@ -45,32 +45,16 @@ docker build \
   --tag run-image-curl .
 ```
 
-### Re-create our builder with `hello-extensions` updated to require `curl`
+### Configure the `hello-extensions` buildpack to require `curl`
 
-Edit `$PWD/samples/buildpacks/hello-extensions/bin/detect` to uncomment the second set of lines that output `[[requires]]` to the build plan:
-
-<!-- test:exec -->
-```bash
-sed -i "14,15s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect
-```
-
-(On Mac, use `sed -i '' "14,15s/#//" $PWD/samples/buildpacks/hello-extensions/bin/detect`)
-
-Re-create the builder:
-
-<!-- test:exec -->
-```bash
-pack builder create localhost:5000/extensions-builder \
-  --config $PWD/samples/builders/alpine/builder.toml \
-  --publish
-```
-
-### Re-build the application image
+Set the `BP_REQUIRES` build-time environment variable to configure the `hello-extensions` buildpack to require both `tree` and `curl` (review the `./bin/detect` script to see why this works).
 
 <!-- test:exec -->
 ```bash
 pack build hello-extensions \
   --builder localhost:5000/extensions-builder \
+  --env BP_EXT_DEMO=1 \
+  --env BP_REQUIRES=tree,curl \
   --path $PWD/samples/apps/java-maven \
   --pull-policy always \
   --network host \

content/docs/extension-author-guide/create-extension/why-dockerfiles.md

@@ -16,7 +16,9 @@ Let's see a build that requires base image extension in order to succeed.
 cat $PWD/samples/buildpacks/hello-extensions/bin/detect
 ```
 
-The buildpack always detects (because its exit code is `0`) but doesn't require any dependencies (as the output build plan is empty).
+The buildpack opts-out of the build (exits with non-zero code) unless the `BP_EXT_DEMO` environment variable is set.
+
+If the `BP_EXT_DEMO` environment variable is set, the buildpack detects (exits with code `0`), but doesn't require any dependencies through a build plan unless the `BP_REQUIRES` environment variable is set.
 
 #### build
 
@@ -57,6 +59,7 @@ Run `pack build` (note that the "source" directory is effectively ignored in our
 ```
 pack build hello-extensions \
   --builder localhost:5000/extensions-builder \
+  --env BP_EXT_DEMO=1 \
   --network host \
   --path $PWD/samples/apps/java-maven \
   --pull-policy always \