Extend proxy support for build & runtime and support non-root users

[emanuelbuholzer]

Thanks for providing an in Docker runnable nist-data-mirror. I ran into two issues (proxy configuration & running as root) when using it and would like to upstream my changes:

• Enable proxy support during build time when running behind corporate proxies using build arguments.
• Extend proxy support during the runtime to be able to run the image without modifications using environment variables.
• Run as non-root user and support random uids to be able to deploy on container orchestration platforms such as Kubernetes and OpenShift.

[stevespringett]

Thanks the PR 👍