tests,util-docker: Add clang-12 to the compiler tests

Change-Id: I07f37df42b370677f4a5f4cd948b7680d551a5ab
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/57430
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

util/dockerfiles/docker-compose.yaml

@@ -103,4 +103,11 @@ services:
             args:
                 - version=11
         image: gcr.io/gem5-test/clang-version-11
+    clang-12:
+        build:
+            context: ubuntu-20.04_clang-version
+            dockerfile: Dockerfile
+            args:
+                - version=12
+        image: gcr.io/gem5-test/clang-version-12
 

util/dockerfiles/ubuntu-20.04_clang-version/Dockerfile

@@ -32,6 +32,7 @@ FROM ubuntu:20.04
 # 9
 # 10
 # 11
+# 12
 ARG version
 
 ENV DEBIAN_FRONTEND=noninteractive