Created new term "Sidecar Container" (#3353)

Signed-off-by: Fernando Harris <[email protected]>
Signed-off-by: Noah Ispas <[email protected]>
Co-authored-by: Noah Ispas <[email protected]>
Co-authored-by: Seokho Son <[email protected]>

Author: 3 people

content/en/sidecar-container.md

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+---
+title: Sidecar Container
+status: Completed
+category: concept
+---
+
+The Sidecar Container is an implementation of the sidecar pattern. 
+An application deployed on a second container will run side by side 
+and share its lifecycle with a primary application running on a main container. 
+
+## Problem it addresses
+
+There are cross-platform situations where it is convenient to group [containers](/container/) 
+and their lifecycle to deal with scenarios 
+like logging, monitoring, tracing, but also security, and networking.
+Before this approach, logging would typically be implemented in the application code 
+inside the container. 
+This can lead to different ways to implement the logic, 
+depending on the developer or application, 
+normally resulting in more complex systems to maintain and govern. 
+For example, updating the logging logic might impact the application in runtime, 
+hence increasing operational risk.  
+
+## How it helps
+
+It enforces the principle of separation of concerns 
+and provides extending functionality without changing the main application's code, 
+by using a separated process running on a secondary container.
+
+The sidecar container will share the same resources,
+including storage and networking, as the main container. 
+While doing this, it frees the main container to be focused on functional tasks 
+or exposing business capabilities. 
+
+For example, when multiple microservices have their sidecar containers 
+acting as logging agents reading from the same directory, 
+the main application is writing its logs to.