chore: set environment variable DD_TAGS for forum

[nadeemshahzad]

Setting this DD_TAGS environment variable so will allow us to enhance the filtering of the forum app in Datadog.

Configuration Pull Request

Make sure that the following steps are done before merging:

• Have a Site Reliability Engineer review the PR if you don't own all of the services impacted.
• If you are adding any new default values that need to be overridden when this change goes live, update internal repos and add an entry to the top of the CHANGELOG.
• Performed the appropriate testing.
• Think about how this change will affect Open edX operators and update the wiki page for the next Open edX release if needed