Improve dev tools for running Docker image

[Benaiah]

• [ ] If possible, fix the build-image's Jenkinsfile so we always push the current release of the xenial branch to latest.
• [ ] Allow passing a tag to start-image.sh either through an environment variable or another CLI argument and have the script start that Docker tag. It will default to xenial.
• [ ] Add two scripts, start-xenial.sh and start-trusty.sh, that pull the specific Docker images netlify/build:xenial and netlify/build:trusty from Docker Hub and run them using start-image.sh.
• [ ] Document all the above in README.md.

See also https://community.netlify.com/t/python-support-needs-to-improve/9617/8. See also https://github.com/netlify/build-image/issues/362 which is relevant to that discussion.