Add build section to index server README

[michael-valdron]

Please specify the area for this PR

index server
documentation

What does does this PR do / why we need it:

Adds instructions on how to build the index server container image devfile-index-base, re-tag it, and push it to an image repository.

Which issue(s) this PR fixes:

Fixes #?

part of devfile/api#1131

PR acceptance criteria:

• Test Coverage
  • Are your changes sufficiently tested, and are any applicable test cases added or updated to cover your changes?

Documentation (WIP)

• Does the REST API doc need to be updated with your changes?
• Does the registry library doc need to be updated with your changes?

How to test changes / Special notes to the reviewer:

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: michael-valdron, mike-hoang
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