Dockerfile.commitchecker: update to be 4.15

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

Dockerfile.commitchecker

@@ -6,9 +6,9 @@ WORKDIR /go/src/github.com/openshift/build-machinery-go
 COPY . .
 RUN make -C commitchecker
 
-FROM registry.ci.openshift.org/ocp/4.14:base
+FROM registry.ci.openshift.org/ocp/4.15:base
 COPY --from=builder /go/src/github.com/openshift/build-machinery-go/commitchecker/commitchecker /usr/bin/
-RUN yum update -y && \
-    yum install --setopt=tsflags=nodocs -y git && \
-    yum clean all && rm -rf /var/cache/yum/*
+RUN dnf update -y && \
+    dnf install --setopt=tsflags=nodocs -y git && \
+    dnf clean all && rm -rf /var/cache/yum/*
 ENTRYPOINT ["/usr/bin/commitchecker"]