[Elastic Agent] Add hint on how to build and test the container for E…

…lastic Agent (#25551)

I had this as a local note and used it often. I thought it maybe worth sharing.

x-pack/elastic-agent/README.md

@@ -0,0 +1,18 @@
+# Elastic Agent developer docs
+
+The source files for the general Elastic Agent documentation are currently stored
+in the [observability-docs](https://github.com/elastic/observability-docs) repo. The following docs are only focused on getting developers started building code for Elastic Agent.
+
+## Testing docker container
+
+Running Elastic Agent in a docker container is a common use case. To build the Elastic Agent and create a docker image run the following command:
+
+```
+DEV=true SNAPSHOT=true PLATFORMS=linux/amd64 TYPES=docker mage package
+```
+
+If you are in the 7.13 branch, this will create the `docker.elastic.co/beats/elastic-agent:7.13.0-SNAPSHOT` image in your local environment. Now you can use this to for example test this container with the stack in elastic-package:
+
+```
+elastic-package stack up --version=7.13.0-SNAPSHOT -v
+```