TF-M: add locales, set to UTF-8

Fix https://projects.linaro.org/browse/LSS-2002
Files are truncated when writing on the filesystem using unicode strings.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: Id16c742f77a6b41c2ba4f3fb56f8c816fff72b43
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index 07f965c..fd71557 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -28,6 +28,7 @@
     libffi-dev \
     libssl-dev \
     libxml-libxml-perl \
+    locales \
     openjdk-8-jdk \
     openssh-server \
     perl \
@@ -55,6 +56,8 @@
 COPY setup-sshd /usr/sbin/setup-sshd
 
 RUN set -e ;\
+    echo 'locales locales/locales_to_be_generated multiselect C.UTF-8 UTF-8, en_US.UTF-8 UTF-8 ' | debconf-set-selections ;\
+    echo 'locales locales/default_environment_locale select en_US.UTF-8' | debconf-set-selections ;\
     apt update -q=2 ;\
     apt dist-upgrade -q=2 --yes ;\
     apt install -q=2 --yes --no-install-recommends ${PKG_DEPS} ;\