Add Hacking documentation

CONTRIBUTING.md

@@ -14,3 +14,5 @@ If after going through the previous checklist you still think you should create
 * Have you tried a non-dockerized Jenkins and get the expected outcome?
 * Output of `docker version`
 * Other relevant information
+
+If you are interested to provide fixes by yourself, you might want to check out the [dedicated documentation](HACKING.adoc).

HACKING.adoc

@@ -0,0 +1,43 @@
+# Hacking documentation
+
+This document explains how to develop on this repository.
+
+## Building
+
+Build with the usual
+
+    docker build -t jenkins/jenkins .
+
+Tests are written using [bats](https://github.com/sstephenson/bats) under the `tests` dir
+
+    DOCKERFILE=Dockerfile bats tests
+    DOCKERFILE=Dockerfile-alpine bats tests
+
+Download the test helpers by updating the submodules
+
+    git submodule update --init --recursive
+
+Bats can be easily installed with `brew install bats` on OS X
+
+
+
+## Debugging
+
+In order to debug the master, use the `-e DEBUG=true -p 5005:5005` when starting the container.
+Jenkins will be suspended on the startup in such case,
+and then it will be possible to attach a debugger from IDE to it.
+
+## How to test the publishing using an overriden target repository on Docker Hub
+
+Create a new dedicated target repository in your Docker Hub account, and use it like follows:
+
+[source]
+--
+export DOCKERHUB_ORGANISATION=batmat
+export DOCKERHUB_REPO=test-jenkins
+# The log below will help confirm this override was taken in account:
+./publish.sh --variant jdk11
+Docker repository in Use:
+* JENKINS_REPO: batmat/test-jenkins
+...
+--

README.md

@@ -261,29 +261,9 @@ By default, plugins will be upgraded if they haven't been upgraded manually and
 
 The default behaviour when upgrading from a docker image that didn't write marker files is to leave existing plugins in place. If you want to upgrade existing plugins without marker you may run the docker image with `-e TRY_UPGRADE_IF_NO_MARKER=true`. Then plugins will be upgraded if the version provided by the docker image is newer.
 
-# Building
+## Hacking
 
-Build with the usual
-
-    docker build -t jenkins/jenkins .
-
-Tests are written using [bats](https://github.com/sstephenson/bats) under the `tests` dir
-
-    DOCKERFILE=Dockerfile bats tests
-    DOCKERFILE=Dockerfile-alpine bats tests
-
-Download the test helpers by updating the submodules
-
-    git submodule update --init --recursive
-
-Bats can be easily installed with `brew install bats` on OS X
-
-
-# Debugging
-
-In order to debug the master, use the `-e DEBUG=true -p 5005:5005` when starting the container. 
-Jenkins will be suspended on the startup in such case,
-and then it will be possible to attach a debugger from IDE to it.
+If you wish to contribute fixes to this repository, please refer to the [dedicated documentation](HACKING.adoc).
 
 # Questions?