This isn't the latest one available on hub.docker.com but it does
match the ID reported by the Xenial builds running on Travis:

  instance: ... travis-ci-sardonyx-xenial-1553530528-f909ac5

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 tests/docker/dockerfiles/travis.docker | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tests/docker/dockerfiles/travis.docker b/tests/docker/dockerfiles/travis.docker
index ea14da29d97..e8eb48dccfd 100644
--- a/tests/docker/dockerfiles/travis.docker
+++ b/tests/docker/dockerfiles/travis.docker
@@ -1,4 +1,8 @@
-FROM travisci/ci-sardonyx:packer-1546978056-2c98a19
+#
+# Travis Image - this is broadly the same image that we run our CI
+# tests on.
+#
+FROM travisci/ci-sardonyx:packer-1552557266-f909ac5
 ENV DEBIAN_FRONTEND noninteractive
 ENV LANG en_US.UTF-8
 ENV LC_ALL en_US.UTF-8
-- 
2.20.1