Bug 1696944 - Part 1 - Update decision image to 18.04 and add an image builder for the decision image. r=ahal,aki

Differential Revision: https://phabricator.services.mozilla.com/D108196

taskcluster/ci/docker-image/kind.yml

@@ -176,3 +176,5 @@ jobs:
     push-to-try:
         symbol: I(push-to-try)
         parent: debian10-base
+    decision:
+        symbol: I(decision)

taskcluster/docker/decision/Dockerfile

@@ -1,5 +1,5 @@
-FROM          ubuntu:16.04
-MAINTAINER    Dustin Mitchell <dustin@mozilla.com>
+FROM          ubuntu:18.04
+MAINTAINER    Andrew Halberstadt <ahal@mozilla.com>
 
 # Add worker user
 RUN mkdir /builds && \

taskcluster/docker/decision/VERSION

@@ -1 +1 @@
-2.2.2
+2.3.0