install clang-tools-6.0 package

The clang-tools-6.0 package contains a 'scan-build' tool, required by
CI scripts.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I8c3155756491f9c024908ca29a8404d3fe6a7bb8
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index 3244660..e6913cd 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -11,6 +11,7 @@
     build-essential \
     clang-tools-6.0 \
     curl \
+    clang-tools-6.0 \
     default-jre \
     device-tree-compiler \
     doxygen \