Allow building and pushing image via GitLab CI

Override these environment variables in "Projects / Variables" section
to push to a private repository:
* DOCKER_REGISTRY_HOST - defaults to hub.docker.com
* DOCKER_IMAGE - defaults to sameersbn/gitlab

.gitlab-ci.yml

@@ -0,0 +1,5 @@
+image: gitlab/dind
+
+deploy:
+  script:
+    - ci/gitlab

ci/gitlab

@@ -0,0 +1,15 @@
+#!/bin/sh
+
+set -e
+
+VERSION=$(cat VERSION)
+DOCKER_REGISTRY_HOST=${DOCKER_REGISTRY_HOST:-hub.docker.com}
+DOCKER_IMAGE=${DOCKER_IMAGE:-sameersbn/gitlab}
+
+docker build -t $DOCKER_IMAGE .
+
+docker tag $DOCKER_IMAGE $DOCKER_REGISTRY_HOST/$DOCKER_IMAGE:latest
+docker tag $DOCKER_IMAGE $DOCKER_REGISTRY_HOST/$DOCKER_IMAGE:$VERSION
+
+docker push $DOCKER_REGISTRY_HOST/$DOCKER_IMAGE:latest
+docker push $DOCKER_REGISTRY_HOST/$DOCKER_IMAGE:$VERSION