Use small/medium public runners

For whatever reason the `docker`/`linux` tags do not appear in the list
of public runners.  Update to values actually present, guessing at their
meaning.

Signed-off-by: Chris Evich <cevich@redhat.com>

.gitlab-ci.yml

@@ -3,8 +3,7 @@
 default:
   image: quay.io/buildah/stable:v1.32
   tags:
-    - docker
-    - linux
+    - saas-linux-small-amd64
 
 workflow:
   rules:
@@ -41,6 +40,8 @@ commit_check:
     fi
 
 build:
+  tags:
+    - saas-linux-medium-amd64
   stage: deploy
   variables:
     BUILDAH_FORMAT: docker