bionic-amd64-tf-a-build: install `gnuplot`

GNUPlot will be used in several jobs generating visualizations of
various project statistics.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I989eb9dd109aa3a32bdd6ec4c646aecb7c34818c
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index ed5cfbd..a19aeea 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -23,6 +23,7 @@
     gcc-multilib \
     gdisk \
     git \
+    gnuplot \
     graphviz \
     jq \
     lcov \
@@ -96,4 +97,3 @@
 
 EXPOSE 22
 ENTRYPOINT ["/usr/sbin/setup-sshd"]
-