Add example on how to make an onbuild image for Perl

Instead of providing an `onbuild` variant, show to the user how it is
created instead, by example.

See https://github.com/Perl/docker-perl/issues/13 and
https://github.com/docker-library/official-images/issues/2076

perl/content.md

@@ -31,3 +31,23 @@ For many simple, single file projects, you may find it inconvenient to write a c
 ```console
 $ docker run -it --rm --name my-running-script -v "$PWD":/usr/src/myapp -w /usr/src/myapp perl:5.20 perl your-daemon-or-script.pl
 ```
+
+## Creating a reusable `perl:onbuild` image for Perl projects
+
+Suppose you have a project that uses [Carton](https://metacpan.org/pod/Carton) to manage Perl dependencies. You can write a `Dockerfile` that makes use of the [ONBUILD](https://docs.docker.com/engine/reference/builder/#onbuild) instruction like this:
+
+```dockerfile
+FROM perl:5.26
+
+RUN cpanm Carton \
+    && mkdir -p /usr/src/app
+WORKDIR /usr/src/app
+
+ONBUILD COPY cpanfile* /usr/src/myapp
+ONBUILD RUN carton install
+
+ONBUILD COPY . /usr/src/app
+```
+
+Building this as a `perl:onbuild` image can let you reduce your project's `Dockerfile` into a single line of `FROM perl:onbuild`, which may be enough to build a stand-alone image for your project.
+