[image-builder-mk3] Add sequence diagram

Author: WVerlaek

components/image-builder-mk3/README.md

@@ -4,7 +4,9 @@
 
 > Once completed, [#7845](https://github.com/gitpod-io/gitpod/issues/7845) will move `image-builder-mk3` (and image builds) from meta to workspace clusters.
 
-See the [Architecture page](https://www.notion.so/gitpod/Architecture-0e39e570b10f4e8ba7b259629ee3cb74) for more details, including a diagram.
+The following diagram shows what happens during a workspace image build. See the [Architecture page](https://www.notion.so/gitpod/Architecture-0e39e570b10f4e8ba7b259629ee3cb74) for more details.
+
+![Architecture](../../docs/workspace/image-builder-mk3/sequencediagram.svg)
 
 Relevant:
 * API definitions can be found [here](../image-builder-api/imgbuilder.proto)

docs/workspace/image-builder-mk3/README.md

@@ -0,0 +1,9 @@
+# Image Builder Sequence Diagram
+
+To edit the diagram:
+1. Go to https://sequencediagram.org/
+1. Open [`sequencediagram.txt`](./sequencediagram.txt)
+1. Make changes
+1. Export as SVG to [`sequencediagram.svg`](./sequencediagram.svg)
+1. Also save changed diagram source to [`sequencediagram.txt`](./sequencediagram.txt)
+1. Consider updating the copy of this diagram on the [Architecture page](https://www.notion.so/gitpod/Architecture-0e39e570b10f4e8ba7b259629ee3cb74#eb026f0404ae42b898a662a665fcef49) as well.