Include mbed-tls host image

The internal Arm Mbed TLS CI uses AMI images as host images that
launches containers to do the testing. For cost and performance
reasons, Open MbedTLS CI would use a different approach: use docker
instead of AMI images, similar to what TF-A and TF-M CIs do.

The proposed docker file defines the image that would act as a
Mbed TLS host image.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: If68a21b2f190436a563fc821350b1deea3ab6365
diff --git a/focal-amd64-mbed-tls-host/build.sh b/focal-amd64-mbed-tls-host/build.sh
new file mode 100755
index 0000000..c026dbf
--- /dev/null
+++ b/focal-amd64-mbed-tls-host/build.sh
@@ -0,0 +1,16 @@
+#!/bin/sh
+
+set -e
+
+trap cleanup_exit INT TERM EXIT
+
+cleanup_exit()
+{
+  rm -f *.list *.key
+}
+
+export LANG=C
+
+image=trustedfirmware/ci-amd64-mbed-tls-ubuntu-host:focal
+docker build --pull --tag=$image .
+echo $image > .docker-tag