docker build for jenkins using proxy for either

• external - tinyproxy (no firewall)
• internal - cntlm (for authenticating proxy behind corporate firewall)

Thanks to starting point provided by

• https://itnext.io/docker-inside-docker-for-jenkins-d906b7b5f527
• https://github.com/smoogie/jenkins_docker_example

Prereqs for external:

• docker installed as host
• user jenkins exists

Should work out of the box.

Prereqs for internal/corporate:

• seperate DOCKER_HOST defined
• set correct env values for DKR_HOST_UID (jenkins) and DKR_HOST_GID (docker)
• set DKR_CNTLM_vars - see env file