bionic-amd64-tf-a-build: Install "expect"

As required to run post-expect scripts now:
https://git.trustedfirmware.org/ci/tf-a-ci-scripts.git/tree/script/expect-post-runner.sh#n15

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I085abf47663795f51815806632728da13a880991
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index 38916c3..08bf54d 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -15,6 +15,7 @@
     default-jre \
     device-tree-compiler \
     doxygen \
+    expect \
     exuberant-ctags \
     file \
     flex \