include file package

The tool 'file' is used in the CI script, so install it in the docker image.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: Id9a2992dabb5c6ef989dcabce0cff417ea25c30d
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index fc067fe..7c8a1c7 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -16,6 +16,7 @@
     device-tree-compiler \
     doxygen \
     exuberant-ctags \
+    file \
     flex \
     g++-multilib \
     gcc-multilib \