focal-amd64-mbed-tls-host: create a workspace mount point

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: If11354f29b61b9962ac0ad2c73324c46559c6f20
diff --git a/focal-amd64-mbed-tls-host/Dockerfile b/focal-amd64-mbed-tls-host/Dockerfile
index ddb72a2..d0a5287 100644
--- a/focal-amd64-mbed-tls-host/Dockerfile
+++ b/focal-amd64-mbed-tls-host/Dockerfile
@@ -43,7 +43,8 @@
     echo 'buildslave ALL = NOPASSWD: ALL' > /etc/sudoers.d/jenkins ;\
     chmod 0440 /etc/sudoers.d/jenkins ;\
     usermod -aG docker buildslave ;\
-    ln -s $(which python3) /usr/bin/python
+    ln -s $(which python3) /usr/bin/python ;\
+    mkdir -p /home/buildslave/workspace
 
 EXPOSE 22
 ENTRYPOINT ["/usr/sbin/setup-sshd"]