TF-M: Set Python3 as the default instead of Python2

Signed-off-by: Benjamin Copeland <ben.copeland@linaro.org>
Change-Id: Ia399392297bf67a135c865cc72dfe36408ef092e
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 057d963..39f4eeb 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -53,6 +53,8 @@
     pip2 install --no-cache-dir -r /opt/requirements_python2.txt ;\
     python3 /tmp/get-pip.py ;\
     pip3 install --no-cache-dir -r /opt/requirements_python3.txt ;\
+    # Set Python 3 as default
+    ln -s -f /usr/bin/python3 /usr/bin/python ;\
     # Setup buildslave user for Jenkins
     useradd -m -s /bin/bash buildslave ;\
     echo "buildslave:$BUILDSLAVE_PASSWORD" | chpasswd ;\