Ensure next-build is able to be viewed on CMS Tugboat

[laflannery]

User Story or Problem Statement

As a engineer on content build and next build, I want to be able to see my work in CMS & Next Build on a Tugboat server so that I can be sure my code changes are functioning as expected.

Issues

1. Next server does not run properly on CMS Tugboat instances (Next Build server does not run correctly on CMS Tugboat instances #20507)
2. Next server branch cannot be changed (Tugboat Next.js server can be restarted via Composer command #17209, Next Build and Vets Website branches can be chosen on Tugboat #16853)
3. There should be a single domain for seeing Next & Content Build pages combined. (Traffic routing for Tugboat #17025)
4. Next Build should reflect Tugboat content changes the way that Content Build is able to. (Next Build should reflect Tugboat content changes the way that Content Build is able to #20543)

Description

Currently we are able to build a Tugboat which shows content-build pages but we are not positive that this is working as expected for next-build changes. As we are prioritizing next-build migration and we anticipate more templates will be on production and more changes will be coming, we will need to make sure we have a reliable way to test changes and updates to the front end when those changes involve next-build pages.

Steps for Implementation

1. This is potentially an issue with node versions being set correctly; see ./tugboat.yml and .tugboat/config.yml
2. Work was done previously for this, and there is an open PR: Tugboat Next.js server can be restarted via Composer command #17400 This should be examined for potential use.
3. This could be an easy nginx implementation, like it is in the reverse proxy. See the logic in the reverse proxy for guidance: https://github.com/department-of-veterans-affairs/vsp-platform-revproxy/blob/main/template-rendering/revproxy-vagov/templates/nginx_website_server.conf.j2#L424

If we need to offer a way to test changed next-build branches with changed CMS branches in the meantime:

1. Set up a CMS Tugboat with your CMS changes.
2. Make note of the URL of your CMS Tugboat instance.
3. In your next-build branch with your next-branch changes, in envs/.env.Tugboat, change the NEXT_PUBLIC_DRUPAL_BASE_URL value to your CMS Tugboat URL from step 2.

Acceptance Criteria

• Next Build server on CMS Tugboat works
• Users can change the Next Build branch within the CMS interface and have the server use their branch code, as is currently the case for Tugboat
• Users are able to hit a single domain to see both Next Build and Content Build mixed together on CMS Tugboat, as is the case on va.gov
• Document the process for how to build a Tugboat that displays next-build pages (e.g. how does this process differ from the well known process folks are used to for building content-build pages)
• Communicate to Sitewide team

[laflannery]

@timcosgrove I created placeholder tickets for the other 2 issues in this epic.

However then I found #17025 and I think it might actually be the same as #20542.

Can you take a look and close either the new one I made or the old one if you want to restart for whatever reason?