Update to environment variables of our Docker image to closely
reproduce the builds run by the Travis-CI service.

Default variables from:
https://docs.travis-ci.com/user/environment-variables/#default-environment-variables

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 tests/docker/dockerfiles/travis.docker | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tests/docker/dockerfiles/travis.docker b/tests/docker/dockerfiles/travis.docker
index b5c5036534..a0e8152317 100644
--- a/tests/docker/dockerfiles/travis.docker
+++ b/tests/docker/dockerfiles/travis.docker
@@ -1,7 +1,11 @@
 FROM travisci/ci-sardonyx:packer-1546978056-2c98a19
-ENV DEBIAN_FRONTEND noninteractive
-ENV LANG en_US.UTF-8
-ENV LC_ALL en_US.UTF-8
+ENV CI=true \
+    TRAVIS=true \
+    CONTINUOUS_INTEGRATION=true \
+    DEBIAN_FRONTEND=noninteractive \
+    LANG=en_US.UTF-8 \
+    LC_ALL=en_US.UTF-8 \
+    TRAVIS_OS_NAME=linux
 RUN sed -i "s/# deb-src/deb-src/" /etc/apt/sources.list
 RUN apt-get update
 RUN apt-get -y build-dep qemu
-- 
2.20.1