chore(ci) build Docker image for each commit to the master branch

These builds are pushed to following Docker repository:
https://bintray.com/beta/#/kong/kubernetes-ingress-controller/master

From #377 

Co-Authored-By: Colin Hutchinson <hutchic@users.noreply.github.com>

.ci/build-master.sh

@@ -0,0 +1,17 @@
+#!/bin/bash -e
+
+if [[ "$TRAVIS_BRANCH" != "master" ]];
+then
+  exit 0
+fi
+
+REPO="kong-docker-kubernetes-ingress-controller.bintray.io"
+export IMGNAME="master"
+export TAG=`git rev-parse --short HEAD`
+
+make container
+docker tag ${REPO}/${IMGNAME}:${TAG} ${REPO}/${IMGNAME}:latest
+echo "${BINTRAY_KEY}" | docker login -u "${BINTRAY_USER}" ${REPO} --password-stdin
+
+docker push ${REPO}/${IMGNAME}:${TAG}
+docker push ${REPO}/${IMGNAME}:latest

.travis.yml

@@ -26,3 +26,6 @@ before_script:
 script:
 - make verify-codegen
 - make test-all
+
+after_success:
+- .ci/build-master.sh