TF-M: set git default config

cppcheck test requires to build mbedcrypto, which fails with a cmake
fatal error: fatal: unable to auto-detect email address

Set a git default config to avoid this error.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: Ice1337777ea294e0b93c573137fa9780c544a864
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 3760a13..07f965c 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -84,5 +84,15 @@
     apt clean ;\
     rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
+USER buildslave
+
+RUN set -e ;\
+    # Set git default config
+    git config --global user.email "ci@trustedfirmware.org" ;\
+    git config --global user.name "TF CI" ;\
+    git config --global color.ui "auto"
+
+USER root
+
 EXPOSE 22
 ENTRYPOINT ["/usr/sbin/setup-sshd"]