Allow frontend to use http proxy

[githendrik]

Hi all,

following a discussion on Slack, I would like to propose the following changes to datahub:

Allow passing of environment variables to the datahub-frontend-react container to make it use a proxy when connecting to external services (namely OIDC providers).

This is my first PR here, so please let me know if I missed anything. I believe the PR checklist is fulfilled, but maybe I missed something else? Happy to change anything not up-to-par!

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub