Document steps to use corporate proxy

[jascott1]

Is your feature request related to a problem? Please describe.
In k8s, behind corporate proxy, build-index container does not pull images from docker.io, seemingly due to lack of http(s) proxy config.

Describe the solution you'd like
Document how Kraken uses http(s) proxies.

Describe alternatives you've considered
I set proxy env vars (upper and lowecase) for build-index container in the k8s manifest but no change.

Additional context

• Manually logging into the build-index container with http(s) proxy env vars set, I can curl http endpoints on the Internet.
• Our container runtime is configured to access the Internet on all nodes and successfully pulls other container images from public and private repos.
• Build-index container logs proper public IPs for index.docker.io (e.g. "2021-05-26T16:37:49.525Z WARN httputil/httputil.go:296 Failed to send https request: Get https://index.docker.io/v2/: dial tcp 3.225.11.9:443: i/o timeout. Retrying with http...")