Add “app container” to glossary (#12611)

* Define "app container"

* Expand app container explanation

* Tweak wording

Co-Authored-By: Zach Arnold <me@zacharnold.org>

* Fix, wording nit

content/en/docs/reference/glossary/app-container.md

@@ -0,0 +1,20 @@
+---
+title: App Container
+id: app-container
+date: 2019-02-12
+full_link:
+short_description: >
+  A container used to run part of a workload. Compare with init container.
+
+aka:
+tags:
+- workload
+---
+ Application containers (or app containers) are the {{< glossary_tooltip text="containers" term_id="container" >}} in a {{< glossary_tooltip text="pod" term_id="pod" >}} that are started after any {{< glossary_tooltip text="init containers" term_id="init-container" >}} have completed.
+
+<!--more-->
+
+An init container lets you separate initialization details that are important for the overall 
+{{< glossary_tooltip text="workload" term_id="workload" >}}, and that don't need to keep running
+once the application container has started.
+If a pod doesn't have any init containers configured, all the containers in that pod are app containers.