Add Dockerfile and custom entrypoint

Dockerfile

+FROM docker:dind
+
+COPY entrypoint.sh /
+ENTRYPOINT /entrypoint.sh

entrypoint.sh

+#!/bin/sh
+
+cd ${CI_PROJECT_DIR}
+docker build -t ${CI_REGISTRY_IMAGE} .
+docker login -u ${CI_REGISTRY_USER} -p ${CI_REGISTRY_PASSWORD} ${CI_REGISTRY}
+docker push ${CI_REGISTRY_IMAGE}