TF-M: add environment variables used by the CI scripts

They can be overriden (per instance) in:
  -> Jenkins configure system
    -> global properties
      -> environment variables

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: I4d76103ec2bdbcf895d0b48ae95dbdb8d56452fb
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index e51ceec..9f961ab 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -1,6 +1,9 @@
 FROM ubuntu:bionic
 
-ENV ARMLMD_LICENSE_FILE=27000@ci.trustedfirmware.org
+# Environment variables used by CI scripts
+ENV DOCKER_PREFIX=987685672616.dkr.ecr.us-east-1.amazonaws.com
+ENV LICENSE_VARIABLE=ARMLMD_LICENSE_FILE=27000@ci.trustedfirmware.org
+
 ENV DEBIAN_FRONTEND=noninteractive
 ENV TOOLS_DIR=/home/buildslave/tools
 ENV PATH=${TOOLS_DIR}/bin:${PATH}