Restore docker build exclusions (#52019)

* Restore docker build exclusions

We have deprecated some old OSes and prevented them from running tests
on the master branch, but that filter doesn't apply to PR branches
pointed at master. This commit restores docker exclusions for those
deprecated OSes in order to allow PR checks to pass for packaging PRs.

Author: williamrandolph

.ci/dockerOnLinuxExclusions

@@ -8,3 +8,8 @@ debian-8
 opensuse-15-1
 ol-7.7
 sles-12
+
+# These OSes are deprecated and filtered starting with 8.0.0, but need to be excluded 
+# for PR checks
+centos-6
+ol-6.10