build/ci: move Dockerfile, document new CI

This moves the Builder Imager Dockerfile into //build/ci, adds some
small changes to make it usable as a Jenkins agent base, documents its
usage, and adds a script which builds and pushes that image into an
external container registry.

We also remove the old Phabricator-based CI scripting.

Change-Id: I332608f7d7105f675104db3ee2d787b2412fcbe9
Reviewed-on: https://review.monogon.dev/c/monogon/+/28
Reviewed-by: Leopold Schabel <leo@nexantic.com>
diff --git a/scripts/create_container.sh b/scripts/create_container.sh
index 2c60e5d..9ddaf7a 100755
--- a/scripts/create_container.sh
+++ b/scripts/create_container.sh
@@ -16,7 +16,7 @@
 fi
 
 # Rebuild base image
-podman build -t monogon-builder build
+podman build -t monogon-builder build/ci
 
 # Keep this in sync with ci.sh: