Use python 3 as the default python. (#1824)
Fixes #1823.
diff --git a/buildkite/docker/centos7/Dockerfile b/buildkite/docker/centos7/Dockerfile
index faee90a..90ca178 100644
--- a/buildkite/docker/centos7/Dockerfile
+++ b/buildkite/docker/centos7/Dockerfile
@@ -154,3 +154,8 @@
popd && \
rm -rf /usr/local/src/tar-1.34 && \
ln -s tar /usr/local/bin/gtar
+
+# Ensure python3.6 (installed earlier) is the default.
+# NOTE: This has to be after every `yum` command because the version of yum in Centos 7 is a python 2 app.
+RUN update-alternatives --install /usr/bin/python python /usr/bin/python3.6 60
+
