Add wrapper for build command

Sample usage:

bin/gitlab-jenkinsfile-runner build --registry registry.gitlab.com --image mbobin/jenkins_runner/cli --build-output ./test-out/

Running build with --registry=registry.gitlab.com --image=mbobin/jenkins_runner/cli --build-output=./test-out/
checking for docker... yes
Logging in to registry.gitlab.com
Username: mbobin
Password:
Login Succeeded
Sending build context to Docker daemon  328.2MB
....