Dockerfile updated from Python 3.8 to Python 3.9

Signed-off-by: Richard Burton <richard.burton@arm.com>
Change-Id: I7cfda89a1eac40a47150505bcacea8bf2aef8695
diff --git a/Dockerfile b/Dockerfile
index 3f5a0a4..75bc927 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -21,10 +21,10 @@
     apt-get install -y \
     make \
     git \
-    python3.8 \
+    python3.9 \
     python3-pip \
-    python3.8-dev \
-    python3.8-venv \
+    python3.9-dev \
+    python3.9-venv \
     unzip \
     curl \
     wget \
@@ -33,6 +33,11 @@
     sudo \
     telnet
 
+# Set Python3.9 as default
+RUN update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 10 && \
+    update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 20 && \
+    update-alternatives --set python3 /usr/bin/python3.9 && python3 --version
+
 # Download and install gcc 11.2
 RUN curl -L https://developer.arm.com/-/media/Files/downloads/gnu/11.2-2022.02/binrel/gcc-arm-11.2-2022.02-x86_64-arm-none-eabi.tar.xz -o gcc-arm-none-eabi.tar.xz && \
     echo "8c5acd5ae567c0100245b0556941c237369f210bceb196edfe5a2e7532c60326 gcc-arm-none-eabi.tar.xz" | sha256sum -c && \