Add documentation for building Logstash custom image with opensearch plugin

Signed-off-by: Florian Medja <[email protected]>

Author: Florian Medja

DEVELOPER_GUIDE.md

@@ -9,6 +9,7 @@
         - [Configuration for Logstash Output OpenSearch Plugin](#configuration-for-logstash-output-opensearch-plugin)
     - [Submitting Changes](#submitting-changes)
     - [Backports](#backports) 
+- [Building Custom Docker Images](docs/building_custom_docker_images.md)
 
 # Developer Guide
 
@@ -215,3 +216,6 @@ original PR with an appropriate label `backport <backport-branch-name>` is merge
 run successfully on the PR. For example, if a PR on main needs to be backported to `1.x` branch, add a label
 `backport 1.x` to the PR and make sure the backport workflow runs on the PR along with other checks. Once this PR is
 merged to main, the workflow will create a backport PR to the `1.x` branch.
+
+# [Building Custom Docker Images](docs/building_custom_docker_images.md)
+

docs/building_custom_docker_images.md

@@ -0,0 +1,79 @@
+- [Building Custom Docker Images](#building-custom-docker-images)
+  - [Logstash 8.x](#logstash-8x)
+  - [Logstash 7.x](#logstash-7x)
+    - [Build Logstash Output OpenSearch Plugin Gem](#build-logstash-output-opensearch-plugin-gem)
+    - [Dockerfile](#dockerfile)
+
+
+# Building Custom Docker Images
+
+To build an image that is not available in the [official Docker repository tags](https://hub.docker.com/r/opensearchproject/logstash-oss-with-opensearch-output-plugin/tags), or to add specific plugins to your image, you can create a custom Dockerfile.
+
+The process varies depending on whether you want to build an image with `Logstash 8.x` versions or `Logstash 7.x` versions
+
+## Logstash 8.x
+
+Create this `Dockerfile` to build an image with `OpenSearch 2.0.2` for **`Logstash 8.x`**
+
+``` Dockerfile
+ARG APP_VERSION
+
+FROM docker.elastic.co/logstash/logstash-oss:${APP_VERSION}
+RUN logstash-plugin install --version 7.1.1 logstash-integration-aws
+RUN logstash-plugin install --version 2.0.2 logstash-output-opensearch
+```
+
+## Logstash 7.x
+
+For **`Logstash 7.x`** , the Logstash output OpenSearch gem needs to be build.
+
+### Build Logstash Output OpenSearch Plugin Gem
+
+1. Clone `logstash-output-opensearch repo` 
+
+    ```sh
+    git clone https://github.com/opensearch-project/logstash-output-opensearch.git
+    ```
+
+2. Checkout the tag for the plugin version you want to build. For the version [2.0.2](https://github.com/opensearch-project/logstash-output-opensearch/tree/2.0.2) for example
+
+    ```sh
+    git checkout 2.0.2
+    ```
+
+
+3. Remove [this line that adds the json version spec](https://github.com/opensearch-project/logstash-output-opensearch/blob/2.0.2/logstash-output-opensearch.gemspec#L49). This version of the JSON gem is incompatible with `Logstash version 7.x`.
+
+4. Build the gem by running the following command:
+
+    ```sh
+    gem build logstash-output-opensearch.gemspec
+    ```
+
+    The Gemfile `logstash-output-opensearch-2.0.2-x86_64-linux.gem` will be generated.
+
+### Dockerfile
+
+Create this Dockerfile to build an image with `logstash version 7.x` and the previously generated `Gemfile` :
+
+```Dockerfile
+ARG APP_VERSION
+
+FROM docker.elastic.co/logstash/logstash-oss:${APP_VERSION}
+
+USER logstash
+# Remove existing logstash aws plugins and install logstash-integration-aws to keep sdk dependency the same
+# https://github.com/logstash-plugins/logstash-mixin-aws/issues/38
+# https://github.com/opensearch-project/logstash-output-opensearch#configuration-for-logstash-output-opensearch-plugin
+RUN logstash-plugin remove logstash-input-s3
+RUN logstash-plugin remove logstash-input-sqs
+RUN logstash-plugin remove logstash-output-s3
+RUN logstash-plugin remove logstash-output-sns
+RUN logstash-plugin remove logstash-output-sqs
+RUN logstash-plugin remove logstash-output-cloudwatch
+
+RUN logstash-plugin install --version 7.1.1 logstash-integration-aws
+
+COPY logstash-output-opensearch-2.0.2-x86_64-linux.gem /usr/share
+RUN logstash-plugin install /usr/share/logstash-output-opensearch-2.0.2-x86_64-linux.gem
+```