Update sentry:onbuild instructions (#558)

* Update onbuild instructions

Hi,

I'm guessing there's a fair chance that the venn diagram of sentry users and docker users doesn't have too much overlap. I've updated the onbuild image documentation to help people get started with a little more information.

I hope you find my additions to the readme helpful.

Thanks.

* doc: Remove README changes

They'll move to content.md

* doc: Move changes in README to onbuild docs

* style: fix markdownfmt warnings

* style: fix markdownfmt warnings again

sentry/variant-onbuild.md

@@ -5,3 +5,11 @@ This image makes it easy to custom build your own Sentry instance by copying in
 It's also possible to develop custom extensions within your `onbuild` package. If the build directory contains a `setup.py` file, this will also get installed.
 
 See the [official Sentry documentation](https://docs.getsentry.com/on-premise/server/installation/) for more information.
+
+To create your custom `sentry:onbuild` package, simply do the following:
+
+1.	Create a Dockerfile containing `FROM sentry:onbuild`
+2.	In the same directory, add your custom configuration files.
+3.	You can get copies of those files to use as templates from the [docker-sentry GitHub repo](https://github.com/getsentry/docker-sentry/).
+4.	Build your image: `docker build -t mysentry .`
+5.	Run your custom image using `mysentry` instead of `sentry`.