Add docker image build via Travis CI

Build the docker image in Travis. Push the result as latest version if
run from the master or next branch. Only push tagged commits from the
master branch as releases.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.travis.yml

@@ -0,0 +1,18 @@
+sudo: required
+
+services:
+  - docker
+
+script:
+  - docker build -t kasproject/kas .
+
+after_success:
+  - if [ "$TRAVIS_BRANCH" == "master" ] || [ "$TRAVIS_BRANCH" == "next" ]; then
+        docker login -u="$DOCKER_USERNAME" -p="$DOCKER_PASSWORD";
+        docker push kasproject/kas;
+        if [ "$TRAVIS_BRANCH" == "master" ] && git describe --exact-match 2>/dev/null >/dev/null; then
+            VERSION="$(/usr/bin/python3 -c 'from kas.__version__ import __version__; print(__version__)')";
+            docker tag kasproject/kas kasproject/kas:$VERSION;
+            docker push kasproject/kas:$VERSION;
+        fi
+    fi