ci: Fixate CentOS container image until rpm-md repos sync

Copy of https://github.com/projectatomic/rpm-ostree/pull/985

Closes: #1160
Approved by: jlebon

.papr.yml

@@ -41,8 +41,10 @@ env:
     CFLAGS: ''
 
 tests:
+    # FIXME revert setting to 7/3/1611 when repos sync
+    # https://github.com/projectatomic/rpm-ostree/pull/985
   - docker run --privileged -v $PWD:$PWD --workdir $PWD
-    registry.centos.org/centos/centos:7 sh -c
+    registry.centos.org/centos/centos:7.3.1611 sh -c
     'yum install -y git && ci/build-check.sh'
 
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